Additive brush when in mask

[JJ1983]

Hi,
when I am painting on the mask I try to get partial opacity using white or black color and reducing intensity but then another and another stroke of a brush is additive. Is there a wait to get gray? Or to paint over the black color but leave some white areas untouched?

[russellcottrell]

To paint with a solid gray color, choose a gray for the brush color and use full intensity.

To paint with gray over black areas and leave white areas untouched, change the Channels of the brush tool to Lighten. Or Darken to paint the white areas and leave black untouched.

http://www.russellcottrell.com/photo/PhotoLine/tips.htm