Cypraea tigris is a very variable species, normally ranging from
nearly white with few scattered spots to heavily marked specimens
where the spots overlap so much that the shell looks nearly black.
Like any normal distribution of a characteristic (the typical "bell
curve"), the farther you go toward the extremes (that is, the farther
you get from "average"), the fewer specimens you find in that part of
the range.  There are more moderately spotted C. tigris than either
lightly spotted or darkly spotted ones.  How "rare" your particular
specimen is would depend on just where it fits into the overall
distribution - in other words, just how melanistic it is.  If it is
dark enough to be in the top 0.1% among all specimens of the species,
it is fairly rare.  If it's only in the darkest 1%, I'd call it
uncommon, but not rare.  And if it's only in the top 10%, I'd call it
"within the darker portion of the normal range".
It's hard to picture your yellow specimen, but one word of caution -
make sure it isn't coated with some kind of lacquer or shellac.
Regards,
Paul M.