Corydora suddenly turned black and has a red tinted belly

[utahfish]

Corydora dont do well on gravel it can wear down their barbels an since they are on their stomachs most of the day the food and waste that gets trapped between gravel increases the probability of bacterial infections to the corydora mainly on their bellies.
Secondly corydora and mollies shouldnt be kept together, corydora are a soft water fish and mollies and other live bearers are hard water fish. Depending on whether you have soft or hard water one of them wont do well.
Third Bettas should not be kept with other fish they should be in solo tanks for their own health and health if the other fish.
Figure out if the water is hard or soft and act accordingly.

 

[seangee]

@Lucy Corbettt - any updates?
What the others said about conditions and substrate etc are correct. Based on your original pic I would not worry about the colour variations. I have (rather a lot of) pepper cories, some are brown, some look grey, some black and some blue. Mine did seem to go darker when I put darker sand in but there is still quite a variety in colour.