Finally something I can answer! I have studied under a traditional animation cel-painter that worked on Disney and Bluth films, I can give some background. When studios wanted to get colors to always match up, there were whole groups of people that spend all day being color scientists. They do lots of research and science to make sure that the paint and colors are always the same by mixing chemicals and making sure to keep the recipe exact every time. When you are painting an animation cel, the technique is to let the blobs of wet paint skim across the plastic. Technically, the brush should never sweep across the surface of the cel to make a brushstroke, the brush is there mainly to push the blobs paint around. Think of it like you are pushing a drop of water softly across a tabletop, you run your brush against the surface of the drop to spread it gently until you finish filling the section you need. I’m not sure how to explain it more, it’s a lot easier to do a demo but hopefully that helps!