The problem with some mapping schemes, is that they can create a poor cognitive economy for some users. In particular, where users are forced to recall the mapping rather than use recognition. Although, using location-based mapping rather than a colour-based mapping would be an intuative control scheme because it simply extends our already existing knowledge and semantic (mental associative) networks. We can easily figure out the paddle on the left is the "A" and the paddle on the right is the "D", etc.
Introducing mutating colour-based mappings (based on power-ups any other mechanic) would result in frustration because some people may experience difficulty reacting to unexpected changes and frequently make mistakes. Contributing to this, users will consistantly have to hold an ever-changing mental model in their short-term memory. Although this can be trained, most people can't pick it up easily. This could be bad because the mantra of good casual games is "Easy to Learn, Difficult to Master".