Hiya Clare, welcome to the forum,
Looking at the list of fish you have you say you have just 4 fish in a 300l tank - or do you have 4 species and a lot of each? Not sure what you mean (I am easily confused
Anyway, you may have a problem having the blood parrot fish and the jack dempseys together as they don't really get on, and neither do severums and rainbow sharks.
It's because some are South American cichlids, and some African, but also they have different requirements regarding water ph.
I would recommend deciding which type of cichlid you want to keep, rehome the other type and then stock our tank with more of the ones you want to keep.
As for the colour change, although some cichlids do normally have colour changes as they mature, it could well be stress related too.
TROPICAL TANK-Elite Tank - 207litres
6 Harlequin Rasboras, 5 Emerald Eye Rasboras, 10 Cherry Barbs, 2 Bristlenose Plecs, 2 Panda Corys. 4 Dwarf Chain Loaches.
ViaAqua Tank AR620 97litres
1 red male betta, 1 BN