With recent and current sets, I noticed that some contain parts of a totally different colour than the rest of the set. An example is 8200-1: Radiator Springs Lightning McQueen, which contains a blue 2x6 plate, 3 yellow 1x2 plates, 2 white 1x2/1x4 angle plates, and 2 grey 1x4 plates, while Lightning McQueen is obviously red.
Parts like these aren't usually visible in the finished model.
Why are they used?
These are parts that should be available in most colours, including red. Did they have an overstock in these colours? Is it to make for easier building? Since this way both the model and the instructions aren't a big blob of red.