Peter is right, all coloured maggots, reds-yellows-greens-discosare dye fed. Only bronze maggots are dyed from whites.
squash a red or a yellow and the stuff that comes out is the same colour, squash a bronze and the insides are white on freshly dyed ones, if they are a few days old then the colour soaks into them a bit
I remember many years ago we had a short period ( in our area anyway ) when there was a shortage of maggots in general and the breeders were only doing whites, we tried various types of dye in the shop i worked in and all were crap for reds. At best you got a light pink colour and the dye tended to only realy take in the creases of the skins