I was working with a file in Photoshop.
First, I turned it to Black and white.
Now I wanted to make her lips red, for which my instructor used the brush tool with the foreground colour as black, which painted her lips red.
My question is why weren't her lips painted black instead of red when the foreground colour is black ?
You were painting a layer mask. So by painting black you are revealing the layer below. Or more specifically in this case you were excluding some pixels from the black and white filter layer, which is conceptually the same thing.
A layer mask is a black and white image where white means include and black exclude, and anything in between means anything in between. This is also sometimes called alpha. There is no color in a mask.
Here the explanation how your instructor (I believe) did it. To do the trick you need two identical layers of the photo. First pict you need to change into b&w and second is original. Choose 1st one and apply the mask on it. Use the Bush tool to paint over the lips. You brush will get only b&w colours. As you there are only 2 options in mask - hide and show. Black is hiding a pict, white is returning hidden back (revealing) . So, when you painting on your mask with black over the lips, you hide them and the layer beneath (still colorful one) just appears and lips there are red.
