IIRC when I was in England the bait shops used food dyes to color their maggots to the color the local fish were biting.
The maggots aren't dyed their food source is, the chicken or whatever meat the maggot farm uses for the blow is dyed therefore when the maggots hatch and start eating they are dyed on the inside, the only exception to this used to be when Chrysoidine was used to colour maggots Bronze as the chemical was the only one to be applied externally, this is now banned as Chrysoidine has proven carcinogenic properties.
But, you are right colour is important, the number of times ive fished using a white maggot and not got a bite, changed to Red and bingo, even at times colour combinations, two white maggots or two red maggots on the hook - nothing, change the hook bait to one white and one red and you start getting bites.
The same applies with other colours too, in the depths of winter ive often struggled using white or red pinkies on the hook, put a flouro one on and you get bites - tis bizarre, same with lures, ive had more fish on lures that have red wool tied around the hook than any other lure, i thinks it's Meps that supply a lot of lures with red wool pre tied to them.
There is no rhymne nor reason why, it just happens, damned frustrating at times though