Final answer:
The Mixer Brush tool in digital painting programs has a reservoir well for the base color and a pickup well for blending colors from the image.
Step-by-step explanation:
The Mixer Brush tool in applications like Adobe Photoshop has two paint wells, the reservoir and the pickup well. The reservoir well stores the initial color that you start painting with, much like having a base paint color on your palette.
The pickup well is used to blend and pick up the colors from the image you're painting on, allowing for a dynamic mixing of colors much in the way traditional paint would blend on a canvas.