I knew instantly what you were doing a drawing of. That's, what? The first colossus you battle? Think so.
The only thing that threw me off was the lack of vegetation 'fur' on it. As such, it looks super naked and one-dimensional. You already mentioned that you wished you had a reference photo when drawing it, so you know. Not a big deal really.
As far as the shadows go, it would have to be a very odd kind of lighting to have those certain areas in shadow. Light-source is in the upper-left behind the head it looks. So looking at it, I would think that the right side of the face would need a little more shading there.
I might be wrong on this, because looking at it know it might also be elsewhere. Hmm.
Eyes are fine to me, because when you finally slay a colossus, don't the eyes go dark? I think they do.
That's all I got right now.