Alexander is correct.
But...
Be sure that you have a backup copy of your working /etc/X11/XF86Config file before you start.
And, know how to restore without X working.
Several of my attempts to control gamma resulted in X not starting. <s>
Each failure was clearly MY error. But, I was sure happy that I'd planned a restore strategy. <g>
When I finally got it right, it was/is very nice.
ck
