Crusader Miniatures has a painting guide for Vikings.
Artizan Designs has a page with very good painting guides for a wide variety of World War II Uniforms.
The New York Public Library has a large collection of color plates of the armies of Finland in the 18th and early 19th centuries.
Here’s a large selection of color plates of the uniforms of Norway and Sweden from 1500 – 1909.
The New York Public Library has an online collection of color plates of Egyptian uniforms for the period 1820 – 1898. This should be a great reference for people interested in the Sudan and other North African campaigns in which Egyptians participated.