A second colour seems like a good choice. You could paint with the right mouse button too. Of course you lose the ability to select objects with right click while you have a paint tool - but as that is already the case with the sculpt tools it is not a problem.
If I add a settings button to the top you could choose to add all of the various useful colour tools - like HSV bars, Swatches, Recently used colours. Or if you are happy with a simpler view you could remove the RGB bars for a smaller panel. Also would be nice to have type in boxes for RGB, HSL, Web colors.
Perhaps there should be 2 versions of a panel, the settings could have 2 columns of tick box options. And a single click on a toggle in the header bar for a panel switches you from 'lite' to 'detailed' controls when you need them. I've used that system before in Fractal Painter seemed to work quite well.
In fact there should be 3 versions of a panel - the other one is just a named title bar where the whole panel is collapsed, but it is still there in the UI for when you need it. Some apps will take the 'minimised' panels and put them in tabs onthe top or down the side. I quite like them minified and staying in place in the docked column.
The other option is to have a button that popups the full control list temporarily so you can tweak the values then let it disappear again. I suppose I can try out both and see what feels best (possibly keeping both methods).