There's an ant on my Rubik's Cube

Perl, 156 143 134 128 127 125 120 119 117 113 109 bytes

Includes +1 for -p

Run with the control string on STDIN, e.g.

perl -p rubic.pl <<< "^^>^^<^^^"

rubic.pl:

@1=wryobg=~/./g;s##$n=w&$&;$y+=$x-=$y+=$x,@1[0,4,2,5,3,1]=@1while--$n%9;@{$n&&--$y%3}[3,0..2]=@1;$1[$n+9]#eg

Explanation

Older version:

@f=gboyrw=~/./g;s##$n=w&$&;$y+=$x-=$y+=$x,@f=@f[2,4,1,3,0,5]while--$n%9;@f=@f[0,$y=1,5,2..4]if$n&&$y--<0;$f[$n+8]#eg

The challenge of this question is to find a coordinate system that makes it easy to track the position and direction of the ant and to still easily get the face identity.

The system I chose was to put standard (x,y) coordinates on the face the ant is on such that the ant is always facing in the negative y direction with the center of the face being (0,0). So:

rotate right: (x',y') <- (-y,  x)
rotate left:  (x',y') <- ( y, -x)  alternatve: 3 right rotations
Step forward:   y' <- y-1

If y was already -1 the ant will leave the current face and step onto the next one. In the new coordinate system there x keeps its value, but y' becomes 1.

This gives an easy coordinate system within a face. I also need something for the faces themselves. There I use an array consisting of

The face to right of the ant            g in the initial position
The face to the left of of the ant      b
The face behind the ant                 o
The face opposite to the ant            y
The face before the ant                 r
The face the ant is on                  w

So the initial array is (g,b,o,y,r,w). Moving to the next face corresponds to rotating the last 4 elements, so moving from white to red makes this (g,b,w,o,y,r). Turning right is a permutation of the first 5 elements giving (o,r,b,y,g,w). Turning left is a simular permutation but can also be done by turning right 3 times, so applying this permutation 3 times. And not turning at all can also be done by applying the permutation 8 times. In fact turning right can also be done by applying the permutation 5 times.

Knowing this the program is rather simple:

@f=gboyrw=~/./g                 Set up the initial face orientation
s## ... #eg                     Process each control string character
                                {this is equivalent to s#.#...#eg because
                                the empty regex repeats the last
                                succesful regex)
$n=w&$&                         Calculate n, the number of right
                                rotations+1 modulo 9.
                                This abuses a coincidence of the control
                                characters:
                                 "<" & "w" = "4" -> 3 right rotations
                                 ">" & "w" = "6" -> 5 right rotations
                                 "^" & "w" = "V" = 0 but that is 9 mod 9
                                 so leads to 8 right rtations

$y+=$x-=$y+=$x,                 This is the same as ($x,$y)=(-$y,$x), so
                                a right rotation of the face coordinates
@f=@f[2,4,1,3,0,5]              Right rotation of the face array
   while --$n%9                 Rotate right n-1 times. After this n=0
                                If this was a step then n was effectively 0.
                                So rotate right 8 times leaving n=-9

    ... if $n                   If a step...
               $y--             ... decrease y ...
             &&$y--<0           ... but if y was already negative ...
@f=@f[0,$y=1,5,2..4]            ... change face and set y to 1

$f[$n+8]                        return the last element (current face)
                                if this was a step, otherwise empty

So for that last statement rotations lead to the empty string and steps forward lead to the current face. Therefore $_ gets replaced by the faces visited on each step.


Brachylog, 287 bytes

:1:2222:"w":"y":["r":"b":"o":"g"]{h""|[L:I:N:A:B:[C:D:E:F]]hhM("^",(NhI,CwX,EY,B:D:A:FZ;AwX,BY,[C:D:E:F]Z),NhJ,(I1,2313O;I2,(Nh2,N$($(O;Nh1,2222O;Nbh1,3223O;3322O);3322N,2332O;3223N,2233O;2233N,3132O;2332N,3231O);IJ,AX,BY,(M"<",[C:D:E:F]$(Z,N$(O;M">",[C:D:E:F]$)Z,N$)O)),Lb:J:O:X:Y:Z:1&}

Expects a string containing the moves as Input, and no Output, e.g. brachylog_main("^^>^^<^^^",_). will write wrrgggy to STDOUT.

Explanation

§ There are 3 types of tiles we can be on: centers (noted 1), edges (2) and corners (3)
§ When we are on a tile, we can denote adjacent tiles in order: front, left, back, right
§ Similarly, we can denote the adjacent colors depending on the current one of the face
§
§ We start on the center (1) of face white ("w"). The adjacent tiles are 4 edges (2222)
§ The adjacent colors of white are red, blue, orange and green ("r":"b":"o":"g")
§ Yellow is opposite of white ("y")

§ We pass those initial conditions in an array, with the sequence of moves as first
§ element, as input to subpredicate 1


:1:2222:"w":"y":["r":"b":"o":"g"]{...}


§ SUB-PREDICATE 1

h""  § If the sequence of moves is empty, terminate the recursion
|    § Else...

§ Here are the variables' names of the input (which correspond to what's described in
§ the first few paragraphs)
[L:I:N:A:B:[C:D:E:F]]

§ If the move is "^"...
hhM("^",

   § The only way we change from one face to another is if the tile we end up on is of the
   § same type as the tile we started from
   (NhI,      § If this is the case
    CwX,      § Then write the color of the face we're facing, this face will now be the
              § current color
    EY,       § The third color in the list is now the opposite color
    B:D:A:FZ  § The opposite color is now the one we face, the color behind us (the third
              § in the list) is the one we were on, and the other 2 don't change

    § If the tiles are not the same type, then we don't change color
    ; 
    AwX,         § Write the current color, this will remain the color
    BY,          § Opposite color stays the same
    [C:D:E:F]Z), § Other colors stay in the same order since we moved forward
    NhJ,              § The new tile type is the one we were facing
       (I1,2313O;     § If we were on the center, then the adjacent tiles are 2313
       I2,            § Else if we were on an edge
         (Nh2,N$($(O; § then if we were facing an edge (changed face), then the new types
                      § of tiles are a double circular permutation of the previous types
         Nh1,2222O;   § Else if we were facing a center, then the new tiles are 2222
         Nbh1,3223O;  § Else (corners) if the tile to our left is the center, then 3223
         3322O)       § Else 3322

       ;              § Else if we were on a corner
       3322N,2332O;   § then one of those 4 possibilities applies
       3223N,2233O;
       2233N,3132O;
       2332N,3231O)

§ Else if the move is NOT "^"
;
IJ,AX,BY,         § We stay on the same type of tile, same color, same opposite color
(M"<",            § if the move is "turn left"
    [C:D:E:F]$(Z, § Then we circular permute the adjacent colors to the left
    N$(O          § we also circular permute the adjacent tiles to the left
;M">",            § Else if the move is "turn right"
    [C:D:E:F]$)Z, § Then we do the same but with right circular permutations
    N$)O)
),
Lb:J:O:X:Y:Z:1&   § Recursively call sub-predicate 1 with the new input, and the next move

Equivalent SWI-Prolog code

If you don't want to bother with Brachylog's compiler, you can run this solution in SWI-Prolog using the following code (this is what gets generated by Brachylog's compiler):

:- style_check(-singleton).

:- use_module(library(clpfd)).

brachylog_main(Input,Output) :-
    1=1,
    brachylog_subpred_1([Input,1,2222,"w","y",["r","b","o","g"]],V0).


brachylog_subpred_1(Input,Output) :-
    1=1,
    brachylog_head(Input, "").

brachylog_subpred_1(Input,Output) :-
    1=1,
    [L,I,N,A,B,[C,D,E,F]] = Input,
    brachylog_head([L,I,N,A,B,[C,D,E,F]], V0),
    brachylog_head(V0, M),
    ( 1=1,
    "^" = M,
    ( 1=1,
    brachylog_head(N, I),
    brachylog_write(C, X),
    Y = E,
    Z = [B,D,A,F]
    ;
    1=1,
    brachylog_write(A, X),
    Y = B,
    Z = [C,D,E,F]
    ),
    brachylog_head(N, J),
    ( 1=1,
    I = 1,
    O = 2313
    ;
    1=1,
    I = 2,
    ( 1=1,
    brachylog_head(N, 2),
    brachylog_math_circular_permutation_left(N, V1),
    brachylog_math_circular_permutation_left(V1, O)
    ;
    1=1,
    brachylog_head(N, 1),
    O = 2222
    ;
    1=1,
    brachylog_behead(N, V2),
    brachylog_head(V2, 1),
    O = 3223
    ;
    1=1,
    O = 3322
    )
    ;
    1=1,
    N = 3322,
    O = 2332
    ;
    1=1,
    N = 3223,
    O = 2233
    ;
    1=1,
    N = 2233,
    O = 3132
    ;
    1=1,
    N = 2332,
    O = 3231
    )
    ;
    1=1,
    J = I,
    X = A,
    Y = B,
    ( 1=1,
    "<" = M,
    brachylog_math_circular_permutation_left([C,D,E,F], Z),
    brachylog_math_circular_permutation_left(N, O)
    ;
    1=1,
    ">" = M,
    brachylog_math_circular_permutation_right([C,D,E,F], Z),
    brachylog_math_circular_permutation_right(N, O)
    )
    ),
    brachylog_behead(L, V3),
    brachylog_call_predicate([V3,J,O,X,Y,Z,1], V4).



brachylog_behead(X,Y) :-
    string(X),!,
    sub_string(X, 1, _, 0, Y)
    ;
    number(X),!,
    number_codes(X,[_|T]),
    catch(number_codes(Y,T),_,Y=[])
    ;
    atom(X),!,
    atom_codes(X,[_|T]),
    atom_codes(Y,T)
    ;
    X = [_|Y].

brachylog_math_circular_permutation_left(X,Y) :-
    string(X),!,
    string_codes(X,C),
    C = [H|T],
    append(T,[H],D),
    string_codes(Y,D)
    ;
    number(X),!,
    number_codes(X,C),
    C = [H|T],
    append(T,[H],D),
    number_codes(Y,D)
    ;
    atom(X),!,
    atom_codes(X,C),
    C = [H|T],
    append(T,[H],D),
    atom_codes(Y,D)
    ;
    X = [H|T],!,
    append(T,[H],Y).

brachylog_math_circular_permutation_right(X,Y) :-
    string(X),!,
    string_codes(X,C),
    append(T,[H],C),
    D = [H|T],
    string_codes(Y,D)
    ;
    number(X),!,
    number_codes(X,C),
    append(T,[H],C),
    D = [H|T],
    number_codes(Y,D)
    ;
    atom(X),!,
    atom_codes(X,C),
    append(T,[H],C),
    D = [H|T],
    atom_codes(Y,D)
    ;
    append(T,[H],X),
    Y = [H|T].

brachylog_call_predicate(X,Y) :-
    reverse(X,R),
    R = [N|RArgs],
    number(N),
    reverse(RArgs, Args),
    (
    N = 0,!,
    Name = brachylog_main
    ;
    atom_concat(brachylog_subpred_,N,Name)
    ),
    (
    Args = [UniqueArg],!,
    call(Name,UniqueArg,Y)
    ;
    call(Name,Args,Y)
    ).

brachylog_write(X,Y) :-
    X = [List,Format],
    is_list(List),
    string(Format),!,
    format(Format,List),
    flush_output,
    Y = List
    ;
    write(X),
    flush_output,
    Y = X.

brachylog_head(X,Y) :-
    string(X),!,
    sub_string(X, 0, 1, _, Y)
    ;
    number(X),!,
    number_codes(X,[A|_]),
    number_codes(Y,[A])
    ;
    atom(X),!,
    atom_codes(X,[A|_]),
    atom_codes(Y,[A])
    ;
    X = [Y|_].

PowerShell, 882 bytes

Usage

Save the code in a script and call it like this from the command line. Assuming working directory is the current directory.

.\WalkingAntcg.ps1 "^^>^^<^^^"

Code

$o=[char[]]"grbowy";[int]$c=4;[int]$global:x=1;[int]$global:y=1;[int]$f=1;[int]$n=5;
$u={$c=$args[0];$1="341504251435240503210123".Substring($c*4,4);$2=$1*2-match".$($args[1]).";$3=$Matches[0];"$3";"012345"-replace([char[]]"$1$c"-join"|")}
function t{param($o,$x,$y)if($o){switch($y){0{switch($x){0{$x=2}1{$y=1;$x=2}2{$y=2}}}1{switch($x){0{$y=0;$x=1}2{$y=2;$x=1}}}2{switch($x){0{$x=0;$y=0}1{$x=0;$y=1}2{$x=0}}}}}else{switch($y){0{switch($x){0{$y=2}1{$x=0;$y=1}2{$x=0}}}1{switch($x){0{$y=2;$x=1}2{$y=0;$x=1}}}2{switch($x){0{$x=2}1{$x=2;$y=1}2{$y=0;$x=2}}}}}$global:x=$x;$global:y=$y}
([char[]]$args[0]|%{switch($_){'^'{$global:y++;if($global:y-eq3){$global:y=0;$c="$f";$f="$n";$z=&$u $c $f;$f,$n="$($z[0][1])","$($z[1])"}$o[$c]}
"<"{$z=&$u $c $f;$f,$n="$($z[0][0])","$($z[1])";t 0 $global:x $global:y}
">"{$z=&$u $c $f;$f,$n="$($z[0][2])","$($z[1])";t 1 $global:x $global:y}}})-join""

Less golfed code with explanation

# Recorded order of cube colours and their indexes
# Green=0,Red=1,Blue=2,Orange=3,White=4,Yellow=5
$o=[char[]]"grbowy"
[int]$c=4   # Ant is currently on this colour
[int]$global:x=1   # X coordinate on this face
[int]$global:y=1   # Y coordinate on this face
[int]$f=1   # Colour that the Ant is facing
[int]$n=5   # Colour beyond that the ant is facing.
# If the ant moves of this cube to the next this value becomes the one he is facing.
# It is also the only colour not neighboring this current colour.

# Anonymous function that will return the colour facing left and right
$u = {
# Cube relationships relative to position. Groups of 4 colours that are important given the order...
# Green=0-3,Red=4-7,Blue=8-11,Orange=12-15,White=16-19,Yellow=20-23
# Get the colours surrounding the current colour we are on and the surrounding ones
# String version: "owrygwbyrwoybwgygrbogrbo"
$c=$args[0]
#  "341504251435240501230123"
$1="341504251435240503210123".Substring($c*4,4)
# double the string so that we can get the characters before and after the facing colour reliably
# Assign the output to surpress a boolean. $2 is not used. Shorter than a cast
$2=$1*2-match".$($args[1]).";$3=$Matches[0]
# Return two values. First is the colours to the left,current and right as a string.
# Second is the colour beyond the one we are facing. If we were to move forward two blocks
# we would end up on this colour
"$3";"012345"-replace([char[]]"$1$c"-join"|")
}
 
# function that will transpose the ants position based on right/left rotation.
# Using current x and y determines what the tranposed values are and return them.
function t{
    param($o,$x,$y)
    # X = $1; Y = $2
    # Left 0 Right 1
    if($o){
        # Right Transpose
        # All values are hard coded to rotate to their new positions
        switch($y){
            0{switch($x){0{$x=2}1{$y=1;$x=2}2{$y=2}}}
            # 1,1 is in the center and nothing changes
            1{switch($x){0{$y=0;$x=1}2{$y=2;$x=1}}}
            2{switch($x){0{$x=0;$y=0}1{$x=0;$y=1}2{$x=0}}}
        }
    }else{
        # Left Transpose
        # All values are hard coded to rotate to their new positions
        switch($y){
            0{switch($x){0{$y=2}1{$x=0;$y=1}2{$x=0}}}
            # 1,1 is in the center and nothing changes
            1{switch($x){0{$y=2;$x=1}2{$y=0;$x=1}}}
            2{switch($x){0{$x=2}1{$x=2;$y=1}2{$y=0;$x=2}}}
        }
       
    }
    # Update global variables with the ones from this function
    $global:x=$x
    $global:y=$y
}
 
# Process each character passed by standard input
([char[]]$args[0]|%{
    switch($_){
        # Moving Forward
        '^'{
        $global:y++
        if($global:y-eq3){
            # We have walked of the colour onto the next one. Update coordinates to the next colour
            $global:y=0
            $c="$f"
            $f="$n"
            # Get the new neighboring colour indexes
            $z=&$u $c $f
            $f,$n="$($z[0][1])","$($z[1])"
        }  
        # Output the colour we have just moved to.
        $o[$c]
        }
        # Turn Left
        "<"{$z=&$u $c $f;$f,$n="$($z[0][0])","$($z[1])"
        # Transpose the ants location by passing current location to the transposition function.
        t 0 $global:x $global:y
        }
        # Turn Right
        ">"{$z=&$u $c $f;$f,$n="$($z[0][2])","$($z[1])"
        # Transpose the ants location by passing current location to the transposition function.
        t 1 $global:x $global:y
        }
    }
}) -join ""
# Line above converts the output to a single string. 

Using a lot of single letter variables used to records the ant current state (colour,position and orientation). The ant is always facing up. When a rotate instruction is read the cube is transposed in that direction. Hardcoded transposition matrices used to determine new position based on current position.

Code satisfies all examples in question.