I was thinking the other day that you see a lot of rabbits that are sort of greyish/brown on green fields and really don't blend in at all with the surroundings. I suppose they are not naturally wild but originally came from escapees but it can't help their cause!

Most other mammals seem to be a fairly good brown colour that keeps them hidden in most of the undergrowth and difficult to see against the background of trees.

Probably because rabbits don't depend on blending in with the background to escape predation, they rely on their speed and being able to get down a hole in the ground instead.

That probably is the reason, although it would make their lives even easier if they are camouflaged.

Also, I don't think there is such a thing as a green mammal (please correct me if I'm wrong). Maybe nature hasn't come up with a way of making green fur?

If you look at animals that are well camouflaged, they fit in well with the environment they live in. While we think of grass as green, when you look at it on a small mammal scale, there is a considerable mix of colours, but predominately shades of brown.

While I've never experienced it myself, I'm told that in the African bush, it is very difficult to see an elephant even from close up, until suddenly you make out a shape, then the thing jumps into sight.

At the rate they breed it's probably just as well that they are easy for their predators to spot. I've always thought of them as nature's answer to fast food

Sloths can turn green under certain conditions, as the fur hosts two species of symbiotic cyanobacteria, which may provide camouflage. The outer fur coat is usually a thick brown, but occasionally wild sloths appear to have a green tinge to their fur because of the presence of these bacteria.

But there are no truly green mammals (nor are there any purple mammals).

Kingfisher

That's very true actually, I'd imagine a green rabbit would blen well within lush grass but in other areas would stand out like a sore thumb.

At the rate they breed it's probably just as well that they are easy for their predators to spot. I've always thought of them as nature's answer to fast food

That's a very accurate way of describing them!

A purple mammal, now that would be something to see!

This is what me thinks, and I hope i'm explaining this right....Our lush green Farm fields are "artificial" and they stay green in the winter, where has in the Winter months most vegetation should turn some shade of brown.

In the Winter most of the cover have died down exposing small Mammals, their brown colour helps them blend in. In the green of the Summer, then the thick vegetation helps to keep them hidden, better escape routes and such. This is why you find a much larger increase in Mammal on Brown field sites to that of Green field.