I am looking for resources that take a comprehensive look at the uniforms used in the German militaries from 1850-1919. Colors, materials used, badge locations, units, etc... This could mean things such as Uniforms regulations form the time similar to our AR 670-1.This could be the German Empire, Prussians, Bavarians, etc... Illustrations would also be a great help.
Osprey Publishing basically makes a living producing such guides for military/historical wargamers.
This link goes to the book for the German Empire 1870-88. It gives you other suggestion relative to it. The books aren't terribly long under 100 pages mostly but come illustrated with uniform examples because the expected purchaser is looking to paint miniature soldiers in the correct colours and uniform.
And usually also include information that puts the subject matter into a military historical context.