What purpose does the colon builtin serve?

The : builtin is also useful with the Bash "assign default values" shell expansion, where the expansion is often used solely for the side effect and the value expanded is thrown away:

# assign FOO=bar iff FOO is unset
: ${FOO:=bar}

It appears the :s in your script are being used in lieu of true. If grep doesn't find a match in the file, it will return a nonzero exit code; as jw013 mentions in a comment, if errexit is set, probably by -e on the shebang line, the script would exit if any of the greps fail to find a match. Clearly, that's not what the author wanted, so (s)he added || : to make the exit status of that particular compound command always zero, like the more common (in my experience) || true/|| /bin/true.

I can think of two places I've used : in the past.

while :
do
     shell commands
     some exit condition
done

That is a forever-loop.

function doSomethingStub {
    :
}

Put in a stub function, just to get top level flow of control correct.

One use I've seen back in the Old Days: Instead of a #!/bin/sh (or whatever) line, you'd see a : line. Some of the older Real Unix kernels or Real Unix shells would use that to mean "I'm a shell script, have sh run me". As I recall this was just when csh was making inroads as a common interactive shell.