Up_colorGetS
_colorGetS returns a litteral string from a Color object. The string can be : "#rrggbba", or "#rrrgggbbba", etc. This format can change if GTK+ 2.x is used instead of GTK+ 3.x.
Prototype : fun [ObjColor] S
- ObjColor : any Color object
- Return : S : the string or nil if an error occurs