I think it would be important for bomb disposal personnel to know the colors of the bombs and relevant markings. A bomb in a field that had not gone off but was dark green would call for a more careful survey, being more difficult to see in green vegetation, whereas a bare metal, or light gray bomb would be easier to spot and mark.
Regarding the photo you posted, it is my guess that the side of the bomb that would be visible to a ground observer would, in some cases, be painted in the field to match the underside color of the aircraft carrying it.
Here's a reference you may find useful:
http://www.google.com/imgres?imgurl=...ed=0CDIQ9QEwAw
Ed