During the mating season males are generally light and greyish in colour, whereas the female is browner, or even red.
The male (on the first two photo's on top) uses his swelled first fingers for better grip on the female during mating.

1st & 2nd photo: In my yard, Hilversum, The Netherlands, 5 April 2012
3rd & 4th photo: Oostvaardersplassen, The Netherlands, 4 August 2011
5th photo: Oostvaardersplassen, The Netherlands, 29 March 2019