R. plumbicolor is stout and viper-like in body structure, and about in total length including tail when fully grown. The eye is moderately large. The
rostral scale is just visible from above. The
suture between the
internasals is as long as that between the
prefrontals or a little shorter. The frontal scale is as long as its distance from the end of the snout or a little longer, as long as the parietals or a little shorter. The loreal scale is as long as deep or deeper, sometimes touching the eye. There are two
preoculars scales and three or four
postoculars. The
temporals are 2 + 3 or 4. There are 7 scales on the upper lip, the third and fourth touch the eye; and 4 or 5
lower labials in contact with the anterior chin-shields, which are shorter than the posterior. The
dorsal scaless are strongly keeled, in 23 to 27 rows at midbody. The ventrals
scutes are 144-160 in number, and the anal is usually divided. The
subcaudal scales are 35-50 in number.
R. plumbicolor is dull green above, uniform or with traces of black markings. Young individuals show an inverted black V-mark on the neck, its apex forwards, reaching to the frontal shield, and a second much smaller one behind, the intervening space being bright yellow or orange; a black stripe from the eye to the angle of the month, and more or less regular transverse spots or cross-bars on the back and tail; belly whitish, yellow or plumbeous, rarely with darkish spots. ==Behavior==