This is so so gorgeous. I love the watercolor shading, and the colors you've chosen. You've done a splendid job on Harry's mouth - it shines, and the generous underlip is both youthful and alluring. Also, I must thank you, since this is a pet peeve of mine, for choosing a believable shade of green for his eyes. So often I've seen artwork in fandom which makes the green so bright his eyes glow radioactively. I also like the thickness of his neck, which adds to the physical sense of him as an adult, and the slight somberness of his expression. This is a Harry who's matured through his experience of war and loss, instead of continuing on unchanged as seems to be the case in canon. He looks wiser here. Quieter.
Then Snape. Healthier than we've ever seen him in JKR's version, better groomed, but worn. His expression is perfect - suspicious and uncertain, unconsciously longing but unable to admit it. I love the way they're giving each other these unspoken sideways looks. And the dark circles under Snape's eyes, the silver in his hair, his sideburns and neat beard, the set of his mouth - incredibly sexy but still intimidating. This is a wonderful double portrait. You've made them so much more real to me, visually, than anybody else.