I am comfortable painting in texture paint mode with standard brushes and masks, however how can I use an image as the brush color? Some reason I am not finding a succinct answer here on the site.
While using a Brush Texture you are painting with the source texture-color color-multiplyed by the Brush color (purple in the case below).
Just set it to Full White (RGB=1,1,1), raise the strenght to 1 and you'll be able to paint the exact colors stored in the texture.
If you need mix the colours of the texture with other textures, you can use the node editor: