Just to sum up a few points:
there are other species of platies in the wild, that is other fish that have the word platy in their English common name (e.g. xiphophorus evelynae)
the ones seen in the lfs are almost invariably either the common platy (xiphophorus variatus) or hybrids between the common platy and the swordtail (xiphophorus helleri) or between the common platy and the xiphophorus maculatus (aka as the parrot platy or Indian platy).
there are lots of different colour variants of the common platy (x. variatus) but all these can breed with each other- just like brown dog can breed with a white dog; they're the same species
several related species - e.g. the variatus, maculatus and helleri- can also interbreed, like a dog can breed with a wolf. It is estimated that the majority of platies seen in the trade have some hybrid blood in them. Hybridisation often but not always resultws in weaker fry. John Dawes (Livebearing Fishes) has some interesting examples where interbreeding different species of limias resulted in fry that were actually stronger and more sexually vigorous than either parent species.
breeders are constantly at work to find new colour versions of the common/mixed platy- this does not mean they will be a new species. Some people are very interested and knowledgeable about this breeding, which of course throws interesting light on genetics (and sometimes creates very pretty fish). Others, like myself, are most interested in the forms and colours that have evolved naturally, to cope with a specific situation in the wild. Sometimes excessive selective breeding can lead to a weakened species, but this depends very much on how it is done.