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I don´t knwo vallejo or andrea paints. But the colour was quite different. WWI was at the beginning a very greenish grey, even with a tinge of ocre; later (1916) more blueish.
http://www.landships.freeservers.com/german_earlywar_uniforms.htm
WWII was more a deep green than a grey, the german word feldgrau is a trap.Later it became more a bluegreen.
use google pictures with "ww1 german uniform", you will find a lot of original items.