A pair of these cobras are summoned during combat with Fangcaller Xorreth, who is only present on days when his world quest, Fangcaller Xorreth, is active. You may have to kill (or feign death and reset) him more than once before your desired colour appears.
Level scaling: Most NPCs will scale with the Hunter's level, within the constraints of their level range. Hunters can tame regular NPCs up to 2 levels higher than them, but can only tame elite NPCs of the hunter's level or below.