How do I get smooth colour gradients on dragon wings?

Gradients on wings and bodies work best when you keep the paint thin and blend while it is still wet. Paint the lighter colour first, then bring the darker tone in along the edge and gently work the two together at the boundary with a clean, slightly damp brush. Build the transition gradually rather than in one heavy pass.

Let each layer dry before judging the result, and keep your brush clean between colours so the tones stay true.