Great puzzler. It's not difficult (there's hints also) but levels get tricky if the pieces are not moved right. Sometimes they get "stuck" on an edge by a tiny bit so it requires an extra move to fix (this is an absoute PAIN on challenge mode).
There was some frustration for the first/second time playing the later levels, but the walkthrough is solid and can be checked if really stuck. Achieving 3+ stars is not impossible either.
But, this may present some extra challenge for colorblind folks, I don't know...
Still, great game! Good to see people had good taste back in '11.