Unlike decimal (base10), hexadecimal (base16) counts 0-9, then A-F (decimal 10-15), in each position; so hexadecimal FF equals decimal 16*2-1 or 255. (Likewise, decimal 99 = 10*2-1.)
Because each component can vary among 256 intensities, all 3 combined (FFFFFF, or 256*3-1) provide 16,777,215 colors. Viewing them accurately depends on your eyes, browser, monitor, and printer. Use this page to see what works, then record the color codes for use in other applications.Each red, green, and blue control displays its color and name, its intensity in decimal, and 3 buttons. The left button [<] darkens by 1 unit. The right button [>] brightens by 1 unit. The center button [xx] displays the intensity in hexadecimal: click to select it, then type a new value on the button face, then click again to change the intensity to the new value.
W3Schools HTML Colors Tutorial
W3Schools HTML Color Names & Codes