We already have a color mixer in the app, if you go to any of the drawing tools it will be the bottom icon where you can customize your pen, brush, marker or pencil settings.
I think this question may be about being able to blend colours together on the canvas with an oil or watercolour style brush, as you can in Corel's Painter app - which indeed would indeed be a great feature.
Ah, well in that case I will make note of that idea. :)
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