James
You are asking a question which indicates that you have not done much reading of either the manual or the Help files. The latter say:
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About foreground and background 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.)
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There is a lot more good information in there too!
Photoshop is not a program where you can just fire it up and go on intuition. You have to make an effort to learn the basics and experiment with what you have learned.
If you then have problems (and you will!) come here with questions.