Hi Piet
Great work on the bunker structure. It looks spot on.
Concerning your question about camouflage it doesn't seem that the Keroman III bunker was painted in any way. It looks to me that the different colours you see in the pictures appear to be just different shades of concrete. As a result of weather influences (rainstreaks), differing drying times, etc.
A bit like 50 shades of grey, but different...
I scanned (and attached) a few images from the Osprey book "U-boat bases and bunkers 1941-45".
I found 2 images of the Keroman III bunker.
The Keroman I bunker and the for example the U-boat bunkers in La Pallice did have some sort of camouflage painted on.