Adam, all experienced painters have dealt with this, so don't think you're alone in voicing this issue. As such, the answers will likely run the gamut from "figure it out yourself" to "there are 72 steps in this procedure, each of which must be followed precisely".
You mentioned a specific historical figure, so the first step is pictures - books or internet. Be careful not to fall prey to "analysis paralysis" - find a decent image that shows the uniform, and keep it (or them) as your primary source. Turning up 200 images via a Google search wastes time you could be painting. Many kits include basic color info for the uniform depicted - decide on your pallet, and start painting.
Next, look for artwork that might show your subject uniform. I don't do your period, but I've often relied on good historical artists (Don Troiani, Keith Rocco, John Griffing) to give me the uniform info I couldn't get via photos (cameras apparently weren't issued during the American Revolution . . .). Again, pick one image, and get painting.
In addition, search the Archives here (and, if you must, on other sites) to see if someone else has painted your figure. You might get lucky and find that someone has provided color recipes for whatever medium you intend to use (acrylics, oils, enamels, Crayola . . .), or might be willing to provide same if asked nicely. If the info's available, start slapping the pigment around. If not, forge ahead anyway, with your best guesses!
Beyond the basic uniform details, you need to develop the ability to paint the generic components of most every figure - eyes, skin, hair, leather, metal. There are tutorials and step-by-step (SBS) articles here for each of these - search them out, and practice, practice, practice.
If there's a modeling store, or club, in your area, make contact and see who is willing to provide some advice. In my experience, most painters are more than willing to chat about what they do. Paint something, and ask for critiques.
You already have an excellent resource at your fingertips - the Planet! Paint something, post an image, and ask questions - the more specific, the better - and see if the responses make sense to you.
Using Malcolm Gladwell's concept of the 10,000 Hours, if this is your first figure, you have much to learn, so GET PAINTING. Some folks start with simple figures and ramp up. Others jump right in with major projects. Regardless of which approach fits you, this much is true - your 5th figure will be better than your first, and your 10th will be better than your 5th, etc.
Uniform details are one thing. Color is a completely different subject. We each see color differently, whether we know it or not. Add to that the vagaries of printing/reproduction processes, plus variations in computer monitors, and myths and/or facts of making/delivering clothes to warriors, and we quickly come to the conclusion that the search for the "right color" can be, at times, a fool's errand.
Find a source (image, print, description), select a medium, and get painting.
Within reason, don't worry what the "Color Police" may say about your choices. Finish the figure, listen to the commentary, absorb any crit that appears valid and incorporate it into your NEXT figure, and disregard the stuff that appears to lack validity.
And lastly, ENJOY the process.
Sorry for the sermon, if that's what it sounded like - that wasn't my original intent. Best of luck, and keep your brush wet!
Nicely put.
