It may not be an 'allergy' but instead a type of sensitivity to it. Her body probably rebels when anything with red dye is injested. Both myself and my son are the same. I never eat artificial ingredients, nor do we allow the kids to. 3 years back the neighbour gave my son a couple red licorice whips...a half hour later he was a different kid...weirding out, hyper, flushed etc. He has never had this reaction since, but had something similar with some candy eaten at the same house the year after. Rules are he eats nothing at this neighbours house unless it is natural.
When he started school, My son was given an allergy chart which we filled out...specifying no chocolate products. 1 Day when he was in kindergarten we got a call from his teacher about his behaviour that day...she had not been able to control him and she could not get his attention. When we had picked him up at the sitters, he was sound asleep...and then lethargic until dinner. I asked what the snack was ...chocolate milk, and chocolate chip cookies...she never read the chart!
From lots of experince I will say that you had better watch what she consumes or she will start having really bad reactions when you are not around. Don't bother having to look for what kind of red dye is in the product, don't give her anything with it in...this includes Jello, popsickles and any unnatural snack. If you need to buy some candy, buy at a GERMAN delicatessan, or health food store. No artificial colours, flavours or preservatives...and they taste better too!
Good luck
B