Go to www.iconico.com and get f'ree ColorPic.
It is a wonderful tool which allows you to click on any area of the screen and
tells you the rgb value. It also allows you to blend and modify colors. I
use it all the time for web design.
AFAIK If you use the same rgb numbers on the same screen you should get the
same color. If you are using two screens they will not match unless you can
tweak them to.