The color is specified by six hexadecimal digits. Hex digits are base 16 digits: 0, 1, 2, ...9, a, b, c, d, e, f. "a" (or "A") is ten, "b" is eleven, ..., "f" is fifteen. Hex "10" is sixteen.

Each of the three monitor colors, red, green and blue, is specified by a two-digit number, from zero (00) through 255 (ff). The digits are two-each in RGB order: #rrggbb.

White (all colors maximum) is #ffffff. Black is #000000. Pure red, green and blue are #ff0000, #00ff00 and 0000ff, respectively. Two-color mixes give yellow (red and green, #ffff00), cyan (green and blue, 00ffff) and magenta (red and blue, #ff00ff). These are the simple colors that delight young children.

Dark colors use very low numbers. For example, this popup's borders are a mix of a bit of blue (60) and half as much red (30): #300060.

Light colors use high numbers. The backgound of the text in this popup is cyan (#00ffff) lightened by mixing in a lot of red: #e0ffff. The purple and yellow stripes in the poster's Attributes list are magenta with a lot of green (#fff0ff) and yellow with a lot of blue (#fffff0).