I'm not too sure, but if you dont mean natural foods, then theres specific crs colour enhancing food that you can buy from the internet like the stuff here. Dunno about shipping out of the UK though, and im sure you will be able to pick them up cheaper somewhere else
From what I have read the colour enhancing food (Japanese) can give an improvement in colour of up to 10% it wont turn an A grade to S grade though, just bring out the white and red more. Really good water quality and a varied diet are probably more important; from the little experience I have spinach seems to bring out the red more in cherry shrimp and crs.