Support setting UI element colors without referencing Slick.Color

Allow  modules to change color values on gui UI elements using NUI.Color.toHex() instead of Slick.Color.
This is only intended as a temporary workaround until NUi arrives.

I also added MAGENTA and CYAN to NUI Color.  Sorry for not making this a separate commit, but these Slick color constants were being used, and I didn't think to do it separately until after the fact.