Photoshop Quick-Tip: Switch Between Foreground and Background Colors
Quick-Tips: There are two keyboard commands that you should know about when using Photoshop. Firstly, the quick foreground and background color reset “D” key (D for Default).
Secondly, the color switcher to make foreground color the background color and vice versa “x” key.
Foreground Color ↔ Background Color
Ctrl + x