I then edited the mask with a Brush creating a sub-mask. When Applying the brush the entire main mask disappears so you cannot see what you are editing. I watched a demo of this process where the presenter was on a Windows machine and this did not happen.
Using the B&W mask, you can check whether the mask is clean or whether further processing is necessary.
In other cases, you may still need a grayscale mask, and the B&W representation is much better for checking, e.g. whether you get a smooth transition when using control points/lines.
