If you bought a color, could a player still change your color on his system?

probably, but it only works for as long as that particular game is open

Yeah, you can still change the colours, and rightfully so I think. For example I'd still prefer to change the colour if we had royal blue vs light blue. Although I suppose as long as premium colours are never similar to one another, Fizzer could force you to change the non premium colours instead, which may mean changing your own colour if you were the light blue in this example. Not sure if it really matters to anyone...

For now though, you can change the colour of someone who has purchased a colour.