I have an article on some german bunkers in Antwerp. These were part of the command post for the LXXXIX corps, one of the corpses supposed to defend the belgian and dutch border. There are some nice present-day pics, and as far as I can tell these bunkers originally had a tan-like colour, so slodder's suggestion seems to be ok. The elements turned these bunkers dark-grey nowadays, and lichens made some of them really green. Of course, that's now, 60 years afterwards. During war this probably didn't happen I suppose.
These bunkers didn't have camo-painting. Of course these are inland commandbunkers, not frontline. But I saw some bunkers on the belgian coast, and I don't think they had camo-painting either. I believe they rather camouflaged it with netting then painting it. But in danmark they did, so I suppose it just depends on circumstances, and camouflaging happened a bit randomly.