If you close your eyes and someone asks you to portray Mickey Mouse. I think the first thing that comes to mind is the vibrant red. It's so iconic that there might even be a special red made just for this color.
The yellow color that follows it is a detail that we see on his shoes.
The other colors are black and white, but that's normal, because this is a character that's almost 100 years old and he first appeared to the audience in black and white productions.