External moniter on a macbook.

I have a question for you guys. I bought a 24 inch dell monitor to attach to my 13 inch mackbook. I bought the dvi to mini dvi converter cable, but i am having problems getting the right color. I can not get the color to match the laptop no matter what settings I use. The color seems to be much too cool. So i have two questions. One if i buy a monitor calibrater will it change the color of both moniters or can I choose between the two. Seems logical you could do anyone you choose, but I want to make sure. Also do any of you use a external monitor on thier macbooks?