The cars back then were green and white, correct. Some years back they switched to silver with green adheseive film to increase the market value of disused cars, and now they switch the green film with a blue one.
The original green car paint was indeed thought as a matter of camouflage. The white was added later because someone thought it would be better idea to have police that shows presence instead of a lurking one...
The German Wikipedia has a diagram of the colours used, it can be found
hereThe northern German States have started to change the uniforms to blue too, the intention is to have a similar appearance of policemen through the whole EU.