At a guess it is an easy to distinguish colour and an associated flavour. Pink milk*, pink ice-cream etc, it is visually clear what it is. The colours associated with other flavours chocolate (brown you are eating/drinking mud), vanilla (white, regular milk or toothpaste), mint (green, mmmm broccoli shake need I say more?). Plus using the same colour all the time helps reduce costs.
*Which as all good Charlie and Lola fans know is a flavour all in itself.