James,
A couple of ways:
#define YELLOW rgb( 255, 255, 0 )
or
public gnYellow
gnYellow = rgb( 255, 255, 0 )
this.BackColor = YELLOW && gnYellow
If you use the #define you can put them all in a .h file to include in your forms and classlibs.
>How would I define a color and use it? I want to be able to substitute the name for the actual RGB command. For example I would like to use thisform.background=Yellow, instead of thisform.background=rgb(255,255,0).