Blending Modes
The Blending Modes menu can be found in the Layers window above where it says Lock (by default, the mode will be Normal). Try creating a new layer above an image, painting over it, and then scroll through the options in the drop-down menu. Each selection will blend the top and bottom layers together in a different kind of way. Darken, for instance, lets only the pixels in the top layer that are darker than those in the bottom layers stay visible.