Step 5: Set The Brush Color To Black
We’ll need to paint on the mask with black to reveal some of the original photo, which means we first need to set our brush color to black. Photoshop uses the current Foreground color as the brush color, so with the mask selected in the Layers panel, press the letter D on your keyboard. This will reset the Foreground and Background colors to their defaults, with white as the Foreground color and black as the Background color. Then, press the letter X on your keyboard to swap them, which sets the Foreground color to black.
You can see the current Foreground and Background colors in the color swatches near the bottom of the Tools panel. The Foreground color (upper left swatch) should now be black: