The Wehrmacht
German Army Uniforms
The "M--" numbers assigned by the reference books refer to the approximate period - despite what some "experts" may say - that these variations appeared. Many people have the misconception that each model was made and issued for that year only, and none prior to or afterwards... i.e., "no M40's were made after 1940 and no M43's were issued in 1944." It is entirely possible for a recruit entering service in 1944 to have been issued an M36 tunic, either from old stocks or one that was remanufactured. (The Germans as well as the US Army, often rebuilt previously worn garments.)
The Germans started the war with what everyone terms an "M36" tunic. All future uniforms were a gradual simplification of this model. There is no evidence that the Germans made any distinction between "models". The service jacket was gradually modified over the years to speed up production. On subsequent models only the changes from the previous one are listed. The years produced were gleaned from dates seen on original uniforms. The principal features of each tunic and observations of production dates on real uniforms are as follows:
Feldbluse M36
In 1936, a new uniform garment was introduced into the Wehrmacht for wear in the field. Collectors usually refer to this as the "M36." The garment evolved from earlier garments developed in the early 1930s, and was actually in use by 1935; earlier field blouses (actually introduced in April 1933) had a field grey collar. The final prewar changes were made to the field blouse in 1936. This final prewar tunic was made from high quality wool with a small mix of rayon, with a full interior lining to reduce wear to the wool body.

M36 tunic without insignia (image courtesy of At the Front Militaria)
How To Paint Wehrmacht "Feldgrau" Uniforms
This is yet another of those many subjects that seem to have as many answers as there are painters. These methods, however, work for me, and give me a German uniform I'm happy with.
I paint my figures from the inside out (and no, I don't bother with eyes), so I will show all the paint guides starting with an undercoated figure that has already had the assorted flesh tones applied. Since everyone has their own favourite for that, I'll restrict myself to saying that I paint the skin areas with GW Dwarf Flesh, put on a coat of flesh ink, drybrush with Dwarf Flesh, drybrush again with Bronzed Flesh and finish up (sometimes) with a final drybrush of Elf Flesh. I don't bother being particularly tidy, mainly as all "messes" are cleaned up by the subsequent layers of paint.
I'll be illustrating my camouflage "how-to" with figures as I would have them when reaching the point that I start painting the camouflaged item.
My main reference for Wehrmacht Field Grey uniforms is based on a number of re-enactors' supply shops. The biggest problem is establishing what colour "field grey" actually is.
There are a lot of questions about wool color. "Field grey" comes in dozens of shades, depending on dye lot, material content (wool versus substitute fibres) and so on. The base color is essentially a mix of grey, green and blue fibers that blend together to yield the color.
The Germans were extremely supply conscious and recycled everything. An M36 tunic made from late war material is not a typical thing, but an M43 made from earlier stocks is entirely feasible. Originals come in numerous shades of green, gray, olive, and even brown. There is no single, correct "field grey" color! There are only acceptable parameters.

Original uniforms, none of which match. The four on the right are all made in 1943. (image courtesy of At the Front Militaria)