First of all, make sure your water is hard and your tank is big enough.
It could be interesting to have 2 variations of a colour and see which type is dominant? Maybe you'll be able to see a genetic pattern which you could use to breed particularly visually outstanding fish. Just don't inbreed too much.
Or you can find a species of Molly or Platy that is rare in your area or very sought after. Then, when the fry are old enough, sell them and maybe get some profit.