Hi Nancy,
Yes and no.  Yes, there are Cypraea tigris in which the dorsum is one solid
color.  However, that color consists of the same pigment that normally forms
the spots (think of it as being so heavily spotted that the spots fuse into
a uniformly colored surface.  Such specimens are quite rare.  Unfortunately,
the purple-top specimens you describe are not rare at all, and also are not
natural.  They are made by buffing or dissolving away the outer layer of
shell, to expose the purple underlayer.  Cypraea caputserpentis and Cypraea
arabica are often sold in this condition as well - in tourist shops, not by
specimen shell dealers!
Regards,
Paul M.