What’s happening to Brighton’s Post Boxes?

What’s going on with Brighton’s post boxes? Here’s the double one one outside the Post Office on Melville Road. And the second was spotted at the Dyke Road end of Buckingham Road.

I thought at first it might be incompetant priming for a new coat. But according to the nice lady in the Post Office it’s not, it’s happened to ‘several’ and seems to be some sort of mischief. Anyone know anything? It’s quite specialist and artless vandalism.