Yeah, that one of the adder peeping out from the behind the stones is a great one.
We've had lots of adders on the north downs this year.
The best way to tell gender is not so much by the background colour but by the colour of the zig zag. Males will have a black zig zag and females a (dark) brown zig zag. The background colour can vary, especially in the males.
Common lizards can be a bit troublesome to identify. Generally, those with spots will be male, and those with stripes will be female. But that's not always the case, but it does serve as a rough guide.
I find slow worms the most difficult to identify. If it has a black stripe down the back it is female, but not all females have this. So if it hasn't then I struggle.
Oh yeah, one other thing. I've been told that the pattern on the back of the head of an adder may be like a finger print and can be useful in identifying individuals. I think this is a current area of research so if you submit your sighting to the local Amphibian and Reptile Group they are particularly pleased with pictures of the animal's head.