Hmm, I thought Klax was funky too, but it has a black + color tab as well. The only thing weird about it is the art on the chip which was unusual for the era (and I think I read it isn't region locked though I've never verified this). I just looked through my games and the only weird one as far as spines on US releases is Side Arms.
As far as CD games go, it looks like there's a bit more variation. There's Fighting Street and Monster Lair which are yellow, a bunch that have the black + color tab, some that are all black (Working Designs), then the later releases that are white + color tab.
On the PC Engine side there seems to be more variation, but that may just be the games I've got.