I've never heard that before.
Personally, I think it's all a marketing gimmick to tell consumers that particular waxes are only for certain colors. Souveran is for dark colors, but yet many people with silver cars use it and love it. Paste Glaz was supposedly for light colors, but it looked very nice on my mom's blue-green Concorde.
Nattys and Nattys Blue are marketed toward certain colors, but I've used both on my silver Protege and never saw any difference between the two.
I haven't tried P21S, but I have read lots of nice reviews on it. Buy some (or S100) and you'll enjoy it.