i'm not going to offer an answer to your question, but it just made me think of michael's disapproval of blue-coloured foods. if you'd heard john saffran on RRR this morning, he made a comment about how kids were more likely to eat blue food than adults because the latter learn to associate the colour with food gone off. so maybe blue m&ms should be phased out then?

Why are the signature colours on salt'n'vinegar flavoured products pink or purple?

This has been in the back of my mind since you posted about it... because I couldn't immediately find the solution by putting the question to google. I investigated a few angles.. psychology of colour, and technical printing restrictions. While I learnt a bit about subtractive and additive colour, and why McDonald's chips taste so good (http://mathaba.net/data/whymcdonalds.shtml interesting read on the construction of flavours).. I came no closer to a solution.

So I mulled it over today. What colour is salt? What colour is vinegar? Do they react with potatoes to make another colour? What colour are potatoes?.. Well, yellow - oh, except for those purple or pink varieties (like the pontiacs). I think this is it. At least I can sleep at night now. ;)

I'll get back to you about the chicken theories later :P