This doesn't look like a Grivet to me, you would expect more of a beard and more white hairs on the black face. A vervet monkey seems the best choice. Do you happen to have a picture that includes the tip of the tail?
@Giant Eland Thanks, though it exactly doesn't show the tip, the tail clearly becomes darker, so another point for Vervet monkey, as Grivets should have a white tailtip.
