The dye, especially if their reds or flouro's (more commonly known as disco's) causes a lot of the maggots to dye, about an extra 10 % more than whites, maggots breathe, and sweat through their skin and the dye can stop this killing them if they take on too much. (the dye is added to the feed before they are taken off, thats why if you squash a maggot you'll see their insides are the same colour as they have been dyed as well, especially evident with reds).
So when you are turning the bait, you come to riddle them to take the turning casters off while they are still a light colour and get them in airtight bags or containers to keep them fresh.If you use dyed bait you get two major problems, on the handriddles you end up with the freshly turned casters and the dead maggots that are killed by the dye, sorting the skins out by hand is immpossible due to the time constraints, customers want a pint of neat casters, they don't want casters with some skins in. The second problem is that dyed bait gives you an immediately dark caster, people like light coloured caster, firstly because they look fresh and also they have the choice then to give them some air and darken them to their own preference.
Its all about presentation and having the best looking, freshest bait you can, a shop's reputation, especially if you have a lot of match anglers, is dependant on the quality of your bait.
I have no problem myself with turning dyed bait for casters for my own personal use as I know they may look dark but they are fresh and I don't mind the odd skin being in there. Customers when they are paying ?2.50 or more for a pint of caster naturally want a good bait and who can blame them, thats why whites are always used.