When I need to mix from one colour to another, I use the gradient tool.
1. Make a shape
2. Add gradient fill
3. Modify gradient ends to be desired start and end colours
4. Set shape aside to use as colour palate: hold ctrl/cmd while using the eyedropper tool to choose any single flat colour along that gradient.
I'd love to see a colour history implemented which keeps track of the colours used, sorted in order of when last they were used.
Bars could be added with stepped gradient swatches between the last two selected colours, or for various colour harmonies. Krita has this functionality built in, and I'd recommend using their implementation to study as a basis for a color mixing/swatch system overhaul.
These additions can be kept out of the way by integrating them into the fill and stroke dialogues.
If anyone would like to see some mockups/proposals for it, let me know. :)
-C