Fixing bug in Random::randomColor when using negative seeds.

It was possible to set a negative seed from python, which caused Random::randomColor to throw an OverflowError. I did this little fix as a pull request because I'm not sure if we should also add a minValue of 0 to the seed plug directly. It's not technically necessary, since the test shows negative values giving consistent results, but negative seeds may be a bit odd from a user perspective.