I don't see any reason why a collar would NEED to be black. Sure if you are into ONLY black leather, black boots, black vest, black collar, etc then I guess that would work to 'keep the theme going' but there is no legitimate reason not to use something fun or cute.
In fact, I think it could almost be sexier to use something adorable and innocuous looking but that is really sturdy and restricting. That jaguar collar looks good, and me and my partner have the purple nylon bondage cords which I absolutely love.
To me, it gets boring to look at the same stuff, same color, all drab all the time. I like bright things, it's interesting. That's the same reason I like colored fishnets or costumes on girls in porn and love when people have tattoos. You see enough naked bodies and it all looks similar. I am a very visual person and love having something to brighten up a scene, and for me toys are no different. Given a choice between something in plain black and something in a color (or especially a pattern!) I will choose the most interesting one every single time.