White is erase.
Certainly the case when drawing on a mask layer, but not on an ordinary image layer.
You should be able to paint with white.
As a test create a two layered document and fill the bottom layer with a dark colour. Then move to the layer above and set the colour to white and draw away. You should see the paint lines, assuming you've set the brush to have a large enough diameter (which you should see an outline of).