Whenever you change the value of a color, you change the color. For example, if you wanted to make a very light red, you could add white, and you would have a color that is more pink then red. If you want to darken that red, you would add black.
In addition to black and white, you can also use any other dark or light color to change the value of the color you want to adjust. This is a great opportunity to make this new color very rich by adding more color, instead of just white or black, to lighten or darken it. Colors tend to be richer and look better when they’re made from many colors.