Here are the two, un-monkeyed-with, German WWII helmets that I have in my uniform collection.
The first, an M35 helmet, originally had the decals on either side (which were carefully scraped off during the war per order) and is a VERY slightly more "mossy" green than the second. (Note also the turned rim and separate grommets around the vent holes.)
The second, an M42 helmet, is a cooler, more "bluish" green color. (Note also the rim is sharp and not turned under ad the vent holes have a stamped reinforcing ring around them.)
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The apparent differences between the interior and exterior colors is caused by the lighting and camera. Each helmet is the same color inside and out.
There were some examples of these helmets that were camouflaged by the users with the standard Dark Yellow, Dark Green and / or Red Brown used on AFVs. These paints, when used (rarely!), were most often hand-brushed on, but there are even rarer original examples where the camouflage was sprayed on.
(Note that ORIGINAL camouflaged German helmets command extraordinarily high prices on the collector's market, so most examples of camouflaged helmets seen today have been faked.)
There MAY have been some end-of-the-war (ca '45) helmets manufactured in Einheits Grau (gray) vice the standard dark green. Einheits Grau was probably the same color as we call "Panzer Gray." I have some original German field equipment with the hardware manufactured in this color, but I've never seen or handled an original helmet painted in this color, only having read it may have happened.
German Army WWII helmets were manufactured in a standard, dark green color.
HTH,