Check out the red viper.
Britain is home to a single adder species, the northern or cross adder (Vipera berus berus), which exhibits sexual dichromatism (the sexes are different colours); the males usually being light grey with black zigzags while the females are brown with dark brown zigzags. Juvenile adders are often deep red-brown in colour and at one time they were thought to represent a different species, a dwarf viper peculiar to Great Britain. Gerald R. Leighton, writing in 1901, even proposed the scientific recognition of the “red viper” as a new species and suggested a name for it.
Chapter 15 of British Serpents is entitled The Small Red Viper and is subheaded: Plea for its recognition as a species – distribution – description – size – venomous – suggested name.
Leighton wrote (p.207):
"The small red viper resembles the common adder in the arrangement of its head-plates and in the number of belly-shields, and is therefore put in the same species. It differs from the adder in most other respects; but the differences, by an arbitrary arrangement, are not regarded as essential. These differences are, however, constant, which to my mind is an all-important point. It has been said that the small red viper is held to be either a variety of the adder or the young of the adder. The latter view is the important one from the point of view of its validity as a species. This opinion presumably is based on the fact that certain ordinary adders exhibit a red colour. It is assumed that these adders, if they could have been examined when young, would have appeared to be small red vipers. But is so happens that this red colour in ordinary adders is characteristic of one sex only, and that the female."
He seems to accept that all the red adders seen were juvenile females but he then tells how, on April 26, 1901, he captured a red adder in central Dorset that was red and most assuredly a male. This snake seems to have led him to conclude that the red adder is a separate species of viper from the common adder. Having obtained additional specimens from other parts of the range, which included much of the common adder’s range in Scotland and Wales, he suggested specific recognition for his new red viper in the following conclusion (p.213):
"If naturalists could see their way to use some specific name, such as Vipera rubra, when referring to this viper, it would greatly conduce to accuracy and clearness, and would avoid the confusion that at present exists."
In truth the red viper is merely the juvenile of the only venomous snake in Britain and is frequently red, regardless of sex, going through an ontogenetic colour change to adult livery later in life.
Sources for more information:
Leighton G.R. 1901 British Serpents. W.Blackwood & Sons. xvi+383pp.