The Art of Color: Nathan Fairbairn and Lighting for Location
While there are many different qualities that a colorist brings to a comic book, one of the most beneficial and subtle effects is the effect a good colorist can have on the line art, shaping a good artist into a great artist and a great artist's work into something transcendent. Unless you're consistently comparing the black & white original pages to the finished color versions, it can occasionally be difficult to accurately assess what a colorist is really doing to change the work.
Thankfully, Nathan Fairbairn is not only one of the best collaborators in the comics industry, he also dedicates time to showing the color theory and thinking that goes into his process on his Tumblr; an indispensible resource for anyone interested in learning more about colorists and comics coloring.