I came across this thread with the same issue. I discovered that it is a masking issue NOT solved by control + click to clear the mask. You have to go to masking dropdown and hit clear.
I found a small brush icon next to the subtool in the subtools dropdown menu that you can toggle on or off. When it's toggled off the whole subtool is darker than if it had been masked but not entirely black and you can't edit at it at all so I guess it's the same as the OP described. I'm as much of a beginner as you can…
I fixed this. Now it's happening again. It seems to have something to do with a background image that was overlaying the model. Somehow a texture is only appearing on top of the model, then when I get rid of the texture the model is dark. I've just spent a bunch of time trying to figure out what I did last time to get my…