Saw the head color in Vault of Glass, it IS brown. Above is the Descendant Goblin.
Hobgoblin:
Minotaur:
Imminent Mind:

UPDATE 6/17/22 - Fixed back plating on Minotaurs and added arm knobs for all variants

UPDATE 6/18/22 - Removed arm knobs due to inaccuracy.

UPDATE 7/28/22 - Adjusted head shading and core colors for all variants. Fixed core armor for Minotaur variants

Source:http://db.destinytracker.com/grimoire/enemies/vex