For brushes you'll need smaller round sables. I use a No 1, 2 and 3 and a 5/0 for details. I have some flat, stiff bristle brushes for dry brushing and for blending when I paint in oils. I also have a soft, flat sable I use exclusively with oils for larger areas.
I used to mix a little black with Humbrol 94, with a touch of white to get a decent base color for Field Gray for Germans. There are as many mixtures for that color as there are people painting though. If your Germans have camo uniforms that will also add more colors to deal with.
Here are Verlinden's mixtures for the colors you're inquiring about. All colors are Humbrol unless otherwise stated.
Field Gray:
Base - Slate Gray 31 + Black 33
Dry Brush - Base + White 34 + French Artillery Green 179
Dark Shadow/Wash - Base + Black 33 (or) Raw Umber Oils
Final Highlight - French Artillery Green 179 + Touch of Grass Green 80
For the Paratroopers you'll need a couple of colors.
US Light Khaki
Base - Khaki Drill 72 + Black 33
Dry Brush - Khaki Drill 72 + White 34
Dark Shadow/ Wash - Base + Black 33 (Or) Raw Umber Oils
Final Highlight - Khaki Drill 72 + More White 34
For the patches on the knees and elbows you'll need a grass green tone. You can use any number of OD tones available from Humbrol, but I'd keep Grass Green 80 close at hand and use it in whatever mix you settle on to keep that light yellow toned green.
Here's something else to consider. In reality, fabric wears out, fades and gets dirty, especially fabric used for combat clothing. Then there are manufacturing variations where one factory uses one kind of dye, while another uses a darker orr lighter dye, or a dye leaning more strongly towards one color tone than the other. What that means is that there is no written in stone formula. That means that as long as the colors you mix look right, they are right. Plus, I assume you'
ll be weathering your figures with dust, dirt or mud. That will effect the colors of the uniform. Just find some good color pics or color plates in a book and try to match the colors as close as possible.