I directly sampled the color of the official art of shadow lugia, but the problem is that it’s clearly meant to be a super glossy, dark pokemon. much more so than normal lugia. It has to be shaded a lot rounder reflections with a lot more intermediate shading colors banded around them.
Love it so far!
My best suggestion is to just outline Lugia’s back and tips of his wings in a lighter color, and maybe the bottom of his feet and the back of his tail in a darker one. If need be, you can add a few lines of lighter color through his body, too, but I’m not sure about that last suggestion. Also, apologies if I’m not doing a good job explaining my idea.