This is a feature that i really miss,as a CG artist,because it considerably helps one get inbetween colors without playing too much with the pallette. Here is what i mean:

Sain Paint Tool ( http://sai.detstwo.com/sai/ ) has brushes which enable the user to mix colors directly on the canvas. I tried to figure out how it does that and came to the conclusion that the brush checks the color you start your stroke on and compares it with the color that is set and comes up with a color value that stands between the two. ( brush stroke color = color that it draws over (same layer) +color that is picked)
Having it check only the color where the stroke starts on gives is a slight smudge effect too,but in a way that the user can actually easily get the transition between two color values (makes shading and lights a bliss)

Another interesting thing is that the brush also tries to emulate thickness of color, not only depending on how hard you press the stylus. That way you get different values of a single color. Ultimately Sai has different brushes that work on the same principle,but their values are differently set in order to emulate real materials ( brush(thick), water brush (less thick), marker,pen..and so on ) It also has a color mixer pallette where you can mix colors,but the users does that on the canvas with the same ease. Having this brush dynamics will enable gimp to have such a color mixer pallette.

This brush is something that i couldnt find in photoshop,i saw that there you have color dynamics,but couldnt get them to work that way. Only Sai Paint Tool and that java applet have a waterbrush, the first one's popularity among graphic artists is rapidly growing just because of that feature.Having it in gimp's brush will make it supperior and unique to other graphic software. This simple feature makes it a lot easier to get right color values and the transition between them directly on the canvas.