BG and FG Colors

Photoshop uses the foreground color to paint, fill, and stroke selections and the background color to make gradient fills and fill in the erased areas of an image. The foreground and background colors are also used by some special effects filters. You can designate a new foreground or background color using the eyedropper tool, the Color palette, the Swatches palette, or the Adobe Color Picker. The default foreground color is black, and the default background color is white. (In an alpha channel, the default foreground is white, and the background is black.)
The current foreground color appears in the upper color selection box in the toolbox; the current background color appears in the lower box. 

To change the foreground or background color, do one of the following:

To change the foreground color, click the upper color selection box in the toolbox. To change the background color, click the lower color selection box in the toolbox. Choose a color in the Adobe Color Picker. 

To reverse the foreground and background colors:

Click the Switch Colors icon  in the toolbox. 

To restore the default foreground and background colors:

Click the Default Colors icon  in the toolbox.
You can use the Adobe Color Picker to select the foreground or background color by choosing from a color spectrum or by defining colors numerically. In addition, you can select colors based on the HSB, RGB, Lab, and CMYK color models, choose to use only Web-safe colors, and choose from several custom color systems.
The Color palette displays the color values for the current foreground and background colors. Using the sliders in the Color palette, you can edit the foreground and background colors according to several different color models. You can also choose a foreground or background color from the spectrum of colors displayed in the color ramp at the bottom of the palette.