How to use random to choose colors

Without using additional imports it's fairly simple:

turtle.colormode(255) # sets the color mode to RGB

R = random.randrange(0, 256, 100) # last value optional (step) 
B = random.randrange(0, 256)
G = random.randrange(0, 256)

# using step allows more control if needed
# for example the value of `100` would return `0`, `100` or `200` only

Rocket.color(R, G, B) ## randomized from values above

Using randomized values of (200,255,23):

_{EDIT: Regarding "would i just change the turtle.colormode() to Rocket.colormode() for the next one?"}

The way I would recommend doing it would be to create a function:

def drawColor():

    turtle.colormode(255)

    R = random.randrange(0, 256)
    B = random.randrange(0, 256)
    G = random.randrange(0, 256)

    return R, G, B

Rocket.color(drawColor())

This way you can call drawColor() anytime you want a new color.

Now that you have the basics of randomizing colors for your drawing you can get quite creative with the values for some awesome looking results (adjust integers to your liking):

#!/usr/bin/python

import turtle, math, random, time

def drawColor(a, b, o):
    turtle.colormode(255)
    R = random.randrange(a, b, o) # last value is step (optional)
    B = random.randrange(a, b, o)
    G = random.randrange(a, b, o)
    # print(R, G, B)
    return R, G, B

def drawRocket(offset):
    Rocket = turtle.Turtle()
    Rocket.speed(0)
    Rocket.color(drawColor(20, 100, 1)) # start (0-256), end (0-256), offset 
    rotate=int(random.randrange(90))
    drawSpecial(Rocket,random.randrange(0, 10), offset)

def drawCircles(t,size):
    for i in range(30):
        t.circle(size)
        size = size - 20

def drawSpecial(t,size,repeat):
    for i in range(repeat):
        drawCircles(t,size)
        t.right(360/repeat)

def drawMain(x, y):
    wn = turtle.Screen()
    wn.bgcolor(drawColor(0, 20, 2))

    for i in range(3): # iterations
        drawRocket(x)
        x+=y
        # print(x)

drawMain(2, 10) # offset, step
input("Press ENTER to exit")

random.choice returns a random element from a sequence, just pass it a list with the values you want to choose from:

rand_color = random.choice(['blue', 'red'])
Rocket.color(rand_color)