Final answer:
Photoshop layer masks work with color, and the color that can be used to hide something on that layer is black.
Step-by-step explanation:
Photoshop layer masks work with color, and the color that can be used to hide something on that layer is black. When you paint with black on a layer mask, it hides the parts of the layer that you don't want to be visible. This can be useful for removing or masking certain elements in an image.