Duncan’s Colonial Modeling blog has a nice series of photos of British uniforms intended as a painting guide for the British in Zululand
The New York Public Library’s Digital Collection has plates of Polish uniforms from 1633 to 1787.
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.
The New York Public Library’s Digital Gallery has a large collection of prints of Austrian uniforms from 1448 – 1896. It’s an incredible painting guide.