Many of my platy offspring have turned out brown/colourless. Was mixing coloured a mistake?

I bought a few platies of different colours for a community tank a while back. After a bad outbreak of ich I lost all but three. One female and two male. One male looks a bit different and has shown no interest in the female, so I expect it was a mollie that has been miss-sold.

The remaining couple seem happy enough. The male is red, and the female is yellow and black. Of the fry that have grown up a bit, the males are red but the females havnt developed colour even after several months. A couple of them are yellow and black like the mother, but most are translucent silvery brown.

For a pretty tank should I buy more stock and remove the colourless ones? Are any colours particularly stable?