Basically, there are four species of mamba: the black mamba, Jameson's mamba, eastern green mamba, and the western green mamba. Of these, the last two are the ones that are often referred to as 'green mambas'.
Though they belong to the same genus, i.e., genus Dendroaspis, one needs to understand that they are two separate species, and are found in different parts of the African continent. While the eastern green mamba (Dendroaspis angusticeps) hails from southeastern Africa, the western green mamba (Dendroaspis viridis) is found in western Africa.
Eastern Green Mamba
The eastern green mamba is an arboreal species, which is mainly found in thickly forested and bush-covered areas of southeastern Africa. Though an arboreal species, it does come down in the pursuit of its prey or to bask under the sun; such instances are pretty rare though. It is also found in coastal scrub, moist Savannah, bamboo growth, and mango plantations. Like all other snakes, the diet of this reptile also consists of birds' eggs, small birds, rodents, frogs, lizards, and small arboreal mammals.
The eastern green mamba is the smallest member of the mamba family. An adult may measure roughly about 1.8 meters on an average. Specimen measuring 3.7 meters have also been recorded in the wild, but on rare occasions. The species has a glossy, grass-green color with a light, bright green-colored belly. It has a slender built, with a distinct head and a long and thin tail. The males of this species are larger than the females.
All mambas are highly venomous; the eastern green species is no exception. Apart from neurotoxins, the green mamba's venom also contains calcicludine and dendrotoxin. The potency of the venom is about one tenth of its infamous cousin, the black mamba. Also, as the snake is relatively small in size, the amount of injected venom is lesser. However, this does not overrule the fatal injury that the bite of this reptile may cause to its victim.
Western Green Mamba
The habitat of the western green mamba is no different from that of its eastern relative. If the eastern green mamba inhabits the evergreen forests of the southeast, the western green is known to inhabit the tropical rainforests of western Africa. It is known to grow up to 1.8 - 2.2 meters―3.2 meters at times―which makes it the largest of the arboreal mambas.
Also known as the West African green mamba or Hallowell's green mamba, this species can be easily distinguished from the eastern one by its large green scales outlined in black. Also, it may bear a variety of colors, ranging from greenish-yellow, olive green, emerald green to sky blue or yellow. Its long tail has yellow scales which are edged in black.
The diet of this species consists of birds, bats, squirrels, small rodents, chicks, and small mammals. A bite from this snake can prove fatal. There have been cases wherein it took less than 30 minutes for the venom to show its fatal effects. Other than this, its venom shares similar features as those of its cousins.
On a closing note, unlike their black cousin, these mambas are shy and reclusive in nature, and prefer escaping any trouble rather than confronting it.