I know just a few things --
A. where you see one, there might be 2-3
B. they have mating / copulating season in May (and especially excitable / dangerous at this time)
C. Color can be different...
this is what wiki has on display: photo of both typical and non-typically coloured viper.
ps. maybe it was not viper but a grass snake after all - but I prefer to keep distance rather than to get closer for more accurate explorations. no kidding with this creature.
That is wisest