Regulations differ around the world.
Many times certain colors are dedicated to certain applications. E.g. red = hot water, blue = cold water, yellow = gas. If you start re-painting the meters, you might get into trouble. If your meters have the basic material color, e.g. brass, plastic, then I don't see much of a problem repainting them.
Attention: some countries the meters are not your property, they stay property of the provider. In that case you should ask.