When using the brush tool on a mask, it is painting black and white rather than burning and dodging
I have a background image, placed a mask on it and an image over it. On the computers at school I can easily select the brush tool and go between black and white as I blend the photos togethers (using it as a less destructive eraser). but at home the brush is actually painting black and white. The setting is on normal, the correct thing is selected,I have even reset the brush settings. I dont know what to do.