a) male; b) - c) females,
variations of livraea; d) male, ventral wiew.
Reference:
From the book "Guida dei Rettili e degli
Anfibi d'Europa", by E. N. Arnold and J. A. Burton, ed. Franco Muzzio & c.
Original title: "Reptiles and Amphibians of Britain and Europe"
(modified).