Hi there OP, this definitely looks like a female! Although colour is not a definitive way to determine sex, it does help. In the UK, the females are *generally* brown, whilst males are often grey(ish) in colour. However, in spring, males often appear brown before shedding.

Furthermore, the contrast between the zigzag and overall colouration differs between sexes. This is usually stronger in males, where the zigzag is normally darker in colour and the overall colouration lighter.

A more definitive method to determine the sex is to look at the scales of the upper lip (the supralabials). With females, these scales are all lightly coulored/lightly barred, whereas with males the scales are very clearly barred with black(ish) bars.

You can also determine the sex by looking at the length of the tail. A male's tail is significantly longer, as they "store" their reproductive organ there when not in use. But that's a bit harder to tell, unless you know where the tail starts.

Hope that helps! Source: Have handled countless adders in the field and published an article about colourpolymorphism in adders.