You know... I don't think they were ALL white. I can't remember how I came to that conclusion, but I noticed that Altered Beast, looking through eBay listings, you can tell by the hinge whether it's a hucard or CD. You can't always tell from the description, because they add search terms that make no sense to widen searches.
but then just when I thought all PCE CD games were white, I noticed that some (consistently) are black. Off the top of my head, forgotton worlds. Is there a pattern, like SCD games are a different colour?