Rule #1: Your models, your paint.
However, late war German Heer uniform colors range from grey, green-grey, gray green, greenish, and grayish. They were never really brown, and certainly never black. Even field uniforms for the Waffen-SS in late war weren't black.
If you are talking about boots, most of what I have read is that by late war most were brown, unless individual soldiers took the time to dye them.