This would allow the user to create mixed color brush strokes, which are very common in traditional painting. When paints aren’t mixed together on a traditional palette 100% thoroughly, the resulting brushstrokes can have a very lively effect.
That would be nice, but you would almost need a 3D brush, wouldn’t you? So that you could tilt it this way and that and dip various parts of it in different colours.
Unfortunately I think it’s beyond the scope of MyPaint for now.
Dieterle’s Fan Brush #1 comes pretty close, but you’ll have to put the colours on the canvas first and then mix them together.