This appeared to be a young monkey as its coat was very light and he appeared quite small and frail. About all I could see from the front was the black cap and in one shot what appears to be dark on the back. The rest of him appeared to be gray. I would imagine his back, tail and outside of the legs would also have been dark brown, but what I saw of its arms was light gray inside and out.
I saw this little monkey in the trees on the outskirts of a jungle village. It was about 10 m up in a tree in the Amazon rainforest of SE Ecuador at 900 ft altitude.
As I watched motionless it seemed to lose interest in me and was distracted by something off to its right, which gave me a good chance to get a couple more shorts, one of which was pretty good before it got scared and scampered off.