Umm, where is the problem?
Yes, you can do that, just like you can mix oil colours on your palette and keep them while you are painting, duh :)
Edit: Basically, you can do whatever you want, as long as it helps you acieve your goal, be it in the canvas, screen, or piece of paper. Don't be insecure in experimenting with practicalities like this.
color swatches window and mix with the color sliders and in Corel Painter, there's even a mixing palette.