If that's their answer gilf then I for one don't believe them! I'm sure that (like most people) I always associate blue with vinegar and green is the colour of cheese and onion.
I think that it is true for all other crisps other than Walkers. When I was a kid Walkers crisps were not available in the South. The two big brands we had were Golden Wonder and Smiths - both of which were blue for S+V and Green for C+O. When Walkers began to dominate the market over the whole of England I became totally confused by the different way they packaged their crisps!!
It's because when you buy a blue packet expecting it to be salt & vinegar and then realise that it's actually cheese & onion you still want the salt & vinegar, so you buy another packet. Clever economics is all it is.