I often run into this frustrating problem when using layer masks.
I have a layer to which I have added a layer mask. Now I select a black brush and paint over it with 90% opacity to hide that part of the layer and reveal the layer below which is grey. If I now paint again over the same area using the same brush, the effect sort of adds up and I now see that area as black instead of grey.
Is there any way at all that I can avoid this? Is there any setting with which the brush will not paint the part that I have already painted?
Rather than using opacity of the brush, you should choose shades of gray for your brushes. That way overlapped strokes will not add up and get darker.
You can also use “Lighten” and “Darken” blend modes on those brushes to avoid messing up other parts of the mask
For example, let’s say you painted part of the mask with pure black, and it’s just how you want it. Now you want to paint some of the rest of the mask with an 80% gray colour. Set the blend mode of the gray brush to Darken, and it will darken anything you paint on that is still white (or ligher gray) but will not affect any parts of the mask that are black (or darker than 80% gray)
If you’ve already done it another way and have a few dark patches you want to fix up, select the color you want with the eye dropper and then paint that color over the darker areas you didn’t want darker.
If there are harsh boundaries between light and dark, you could even try blurring the mask with gaussian blur to smooth the transition.