Les Uniformes de l’Armee Francaise contains a series of marvelous images that will work very well as a painting guide.
Painting Guides
British In Zululand Painting Guides
Duncan’s Colonial Modeling blog has a nice series of photos of British uniforms intended as a painting guide for the British in Zululand
Polish Uniforms 1633 – 1797
The New York Public Library’s Digital Collection has plates of Polish uniforms from 1633 to 1787.
Guide To Painting World War II Germans
The League of Augsburg website has a pdf quick and dirty guide to painting World War II Germans. There are some good suggestions in here as to which Vallejo and other paint lines to use.