This color selector resembles the little water cup you use for blending colors when you're making a watercolor painting.
If you want to mix a color, simply move the mouse over the color scale while pressing the left mouse button. If the color gets too dark you just press the right mouse button to fade the color. You can also set the pressure with which you collect the colors. Setting a low pressure means you have to drag more to get a dark, highly saturated color, but gives better control over the mixing.
You can save ten colors in the dialog. If you want to save a color that you have mixed, just press new and the color will appear in the color array. Remember that you can only save ten colors. If you already have ten colors and save a new color, the oldest one will be replaced.