The color brush is used on images where not quite enough green has been removed. This usually happens around the contour of red hair and on sheer material.
Running the Color Brush Prep action will select the Color Replacement Tool with the appropriate settings and also put you on the Tint layer. After running the action, you will need to set the foreground color to the color you want to paint with and set the background color to the area still containing green. You can then paint over the green area.
Once you are happy with the result, play the Color/Spill Brush Done action to return working on the green mask.