When mixing colors, especially if one or more of the pigments are made from natural substances (as opposed to synthetic dyes), the colors will partially separate as the binding medium sinks into the paper and evaporates. You can’t control a lot of this process. By experimentation you can have a good idea what the colors will do when mixed and applied in a certain way, but it’s like a chemical reaction. You wind it up, let it go, and the end result is a splendid surprise!
Over the weekend I took a watercolor class, below are a few selected swatches that I found particularly pleasing and exemplary of what I’m talking about. You’ll see a lot of cerulean blue, yellow and violets (made by mixing cad red or crimson with the cerulean), I think I was just in a mood and loving those colors together. Enjoy!