The dyes are often a mixture of diazo dyes, including Sudan IV. This is banned in food stuffs as it is cancer causing. Its classed as OK in maggots as it is inside their skin and should not come into contact with humans in any concentration. Obviously this is not the case for anything eating the maggots. UK fishing maggots are usualy blue bottle (blowfly) maggots, and I would be very wary of using them as food, though I have used the flies with tree frogs.