In life and politics, itís sometimes hard to find common ground. Fortunately in the world of Photoshop and PSE, itís easier to find those grounds: foregrounds and backgrounds.
Using the Eyedropper tool, clicking once selects a new Foreground Color. Alt-clicking (Mac: Opt-click) captures a color for the background.
No matter what colors you have selected, pressing the letter D on your keyboard resets the foreground color to black and the background color to white. Pressing the letter X on your keyboard switches the foreground and background color.