Make color constants instances of the Color class from the colour module

[amandakrohn]

From the original issue:

Seeing how the natural trend is to go towards convenience, I think they should be defined as constants. Furthermore, they should be instances of the Color class from the colour module (or @marcin-serwin's fork).

The Colour module

[amandakrohn]

This issue was fixed in PR #9, will close.