I already fixed the Twain photograph (Both of them
http://i.imgur.com/tnaAu.jpg -
http://i.imgur.com/j0ohU.jpg), I don't mind revealing my entire process. Do you know how Photoshop works? If you don't then it's a fairly complicated process for a computer illiterate person, no harm intended.
Essentially photoshop adds layers of colored light onto the image. Essentially the reason why images appear brighter and more alive when color is added to them is simply because they're plastered with colored light. The way I do it is judge by eyesight before I start anything and pick out my colors with a notepad file I have stored where color (hex decimals used to identify and find colors) codes are within. So I add 5-6 layers for each image (Usually for Union and Confederates I add one for Skin, Hair, Eyes, Uniform, and Brass objects.), and I assign skin color code to the skin layer (obviously), and then color the designated area, and I ctrl-click the layer (which highlights the color inside the layer and everything else is removed so I can tell if I have areas that I've missed.), I do this for all of them, then I adjust the lightning on the image so it isn't overly bright or overly dark either. Once you get in to it it's a pretty simple process that can take anywhere from 5 minutes to several hours on end.