Fix parsing color as sequence of byte ints

[Conengmo]

Closes #190.

Fix the issue: color components that are byte values between 0 and 255 should be converted to normalized floats between 0 and 1.

I noticed our code here is quite sparse. When addressing this code, I strive to add more checks and more meaningful error messages. Downside is that it makes the code more verbose, but I hope the explicitness is helpful.

I also added tests for the other parsing related functions.