Those colors are extremely beautiful, especially with the reflective light on the rock. It makes it look like the water's glowing, and makes the painting look mysterious and also beautiful. I also love the use of monochromatic colors on the siren herself. Personally though, I think her face looks kind of "smooshed" together (for lack of a better word) and her forehead is kind of long, but I think it still works because it makes her look more monster-like. I've never actually played Witcher though, so my apologies if they really do look like that. :P
Overall, I'd say this looks really nice. :>