When you use the mixer brush to sample a piece of paint, its impossible to replicate that sample when you paint over it. Thats why a mixing pallete could be a good idea. Its essentially a regular photoshop canvas within your tools, but picking a color wouldnt require another 'click' to get within the canvas. an option to save a pallet or have several would be nice as well.
I'm not sure if such an option already exists, because if so, i'd love to have someone tell me : )