Hint:
$R == $G == $B
What Jaydee is saying is that if you set $R (red) , $G (green) and $B (blue) the same, you will get a grayscale object, because equal amounts of those three colours equals a shade of gray. (example if you have full red, full blue and full green you get white, if you have 0 red, green and blue, you get black.