WIP black dragon, need advice on where to darken for better definition.

Have you done a wash already? If so another coat (just on the wings) may help.

A possible method to help you identify where to apply darker shades and washes would be to stand the mini in its designated pose, shine a light source on top of it, then take a picture of the mini from all angles. Then use those pictures a reference to paint the areas in darker shades and washes.

To me this doesn't look like it needs to be darkened, it needs more contrast. Take a light toned brown to the wing skin as a base, and then hit it with a black wash all over, and then a second wash near the arms of the wings so it gets lighter the further away from the arms.

For the green scales, hit it another black wash, and then brighten by making lines of the green.

Painting it for hubby who prefers his minis to match the monster manual, but I would like there to be more definition specifically between the wings and the wing joints.

here’s my dragon with zenithal highlights, if it helps!

Edit: Also, gave you tried lining? The above photo doesn’t include lining.

I'd give a drybrush or something with a light grey to the centre of the wing membranes, then another wash of black right up next to the fingers of the wings. That should give you a nice gradient I reckon!