My daughter (5) reacts to some dyes within an hour (the worst being whatever they put in azythromycin to make it pink), and the reaction (anger, meltdowns, inability to cope) lasts about 48 hours--imagine giving that to her for five days when she's already sick on top of it. When my son was little (now 25) anything purple made him break out in a rash within 24 hours. We recently discovered the effects of evil BHT. The reaction doesn't begin until about 24 hours later, then lasts another 48 hours. We had to strictly keep a food diary for several weeks to determine all this, which is difficult, especially if your child attends school or day care. To sparklepanda I would add that it would be worth investigation salycilate sensitivy in addition to food dyes because the three foods you mentioned are very high in salicylate. If you want to investigate, I would recommend Feingold.org.