This color was all over the place, and things were made worse by the fact that, especially in the early and mid-war periods, the Germans used millions of captured uniforms to make uniforms for the Wehrmacht. Go to a militaria show if you can, and see the differences for yourself. There were Dutch, Polish, French, British, and Russian uniforms reworked or taken apart and recut (and redyed). Early war (M1936) German uniforms also usually used earth brown trousers with the field gray tunic, though the trousers were field gray later. However, unless the two parts of the uniform were made from the same dye lot, the uniform would not have matching parts. That said, it is far better in modeling to stick to the verities, that is, use colors that are close to the standards. However, don't use the same batch of field gray for all uniforms; mix the colors, and understand that many German uniforms were not field gray. Many of the HBT fatigue uniforms were vaious shades of green. Uniforms are perhaps the one area where you CAN match colors (roughly) to good photos of preserved clothing. The colors in good photos will be close enough to represent uniforms pretty well. I have acquired just a few books (most from France) that are of models wearing preserved WW2 German uniforms of all the Wehrmacht services. I am confident they will allow me to paint figures with acceptably accurate colors. It's best not to make this too complicated: folks who insist uniforms can be only the "standard" color do not know what they are talking about. Don't go too far afield, but do NOT sweat the colors that much.....
