The Power of Gray - The Photoshop: Professional Retouching and Compositing Tips, Tricks, and Techniques - page 143

Figure 2.70
7. Press D to set the foreground and background colors in the toolbar to their default
of black and white. Click the black foreground color to open the Color Picker, and
set H to 0, S to 0, and B to 50. Setting B (Brightness) to 50 gives us 50% gray
( Figure 2.71 ) .