The best reference books are any of the Osprey series of books also this is a good reference book The Illustrated Encyclopedia of Uniforms of World War I: An expert guide to the uniforms of Britain, France, Russia, America, Germany and Austro-Hungary with over 450 colour illustrations https://www.amazon.com/dp/0754823407/ref=cm_sw_r_cp_apa_JQkLyb361JMT6
The Osprey series looks like this
The French Army 1914-18 (Men-at-Arms) https://www.amazon.com/dp/1855325160/ref=cm_sw_r_cp_apa_tUkLyb7TEWWS3
Another thing to consider is that there were great variances in shades of colors between one textile manufacturer to another. Especially at that time when they were under the gun to churn out uniforms as fast as possible. Like the French "Horizon Blue" uniforms would vary from manufacturer to manufacturer. Some had more of one fabric dye than another and did the best they could to meet army color specifications. Plus some uniforms, depending on age, were more faded than some others even in the same unit. As long as you're in the ball park it should be considered realistic. I think if you have multiple figures of the same type in a scene their uniforms should have a little variance due to all these factors.