There are only white, black, red, green, blue, cyan, magenta, and yellow. How can I add others? (I would like to use Green (different from green).)
xcolor, so you can use the coloring system of this package, which is very powerful.
For instance, you can use
green!40 etc., for different depths of green or even to mix it with other colors for ever more green colors. For example,
green!70!black!30, etc. You can mix as many colors as you want.
! symbol which separates colors and color depth. You can also define your own colors. For more information use the documentation of the
xcolor package, as Ingmar mentioned in a previous comment.
LaTeX can help you with an infinite combination of colors—not limited to only VIBGYOR primary. Just head on over to LaTeX Color Definitions and see the endless possibilities. A snapshot is below: