How can I make my turkey gravy look darker and richer?
As @Eric Hu notes, a dark roux is the way to go. It's interesting that he mentions Alton Brown, as it's his turkey gravy recipe I use. His recipe also uses red wine, which further darkens the gravy, richens it, and adds a fantastic flavor. I'd only change one thing: next time I'm going to make the roux and finish the gravy in a separate pan after deglazing. The roasting pan is too big and unwieldy.

As noted in Eric's answer, a roux is just flour cooked in fat until it turns the color you want. I usually end up a little darker than here before adding the liquids back to the pan:



I probably wouldn't go this dark unless making gumbo: