Alan,
they need carotenes adding to their diet. I've heard of people adding pure canthaxanthin to their food- I'm told this works very quickly and effectively.
There are various foods for fish and birds which have added carotenes to improve colouring- I've wondered whether feeding livefood on these would improve Bombina colour.
I think there's even a herp colour food from Nekton, though I've never actually seen it for sale.
Another way is to feed livefoods which are already high in carotenes- Daphnia and Gammarus are the best known ones- I've had very good results feeding B. variegata with Gammarus.
This applies to newts as well- CB alpine newts, for instance, often have yellow bellies, where wild ones can be deep red.