The Power of Gray - The Photoshop: Professional Retouching and Compositing Tips, Tricks, and Techniques - page 102

Figure 2.25
2. Set the foreground and background colors to their default of black and white by
pressing D. Click the foreground color in the toolbar.
3. In the Color Picker, set H (Hue) to 0, S (Saturation) to 0, and B (Brightness) to 50
( Figure 2.26 ) . This gives you 50% gray. Click OK.
Figure 2.26