telling a platy apart from a molly

I think color and finnage are the best identifying features--though there are an increasing number of colors being developed with hybrids. Mollies are normally black, white, silver, or marbled, with some yellow/orange showing up in the hybrids. There are several types with fin variations (lyretails, sailfins, balloons).

Platies are based on the wild stock that is red with a black patch along the ventral surface--and I have never seen platies that are not brightly colored in part. Most variations of platy have moved away from the black patch (mickey mouse are the exception), and come in shades of orange to red to yellow. They will not have fin extensions and are normally smaller and more compact than mollies.

HTH

Actually, dawn and blue platies are some fairly common non-bright platies.


The difference in body shape and length generally gives them away though, with the possible exception of balloon mollies (but they honestly don't look very attractive at any rate IMO)