I was watching some documentation on the reasoning behind the styling with hair color, eye color and hair style, and it was originally seen as a way to easily tell characters apart on the screen. It made it really easy to say, okay, that's this character, and this is this character. In a culture where students will wear uniforms, and when reflected in different mediums will also wear uniforms, and easy way to identify everyone individually on screen was born.
Mixing paints is cheap, which is why there is a pretty wide variety of skin tones in animation. Generally though, you write down what you're mixing, and if it looks great, you share your formula with the rest of the animation crew, so they can help keep it up. Of course, digital painting is even easier, because you only have to share 18 character hex codes.