How does LEGO decide the color of internal set pieces?
Going for contrast to help you build?


This is probably the primary reason most of the time.

One Eurobricks user once said they asked a set designer about it, and they confirmed they do it to makes it easier to follow the instructions:


  I had an opportunity some time ago to talk to a designer at TLG, and he told me that they use other colors to make it easier to follow the instructions and build the model.