I have a small lake surrounded by environmental texture with a lot of trees. And my problem is the trees are making the lake surface more greenish then I would like. So my question is - is there a way to add some blue light to the environmental texture node tree? Or it will be better just to add an area lamp with bluish light above the lake?
you could tweak the texture with rgb curves, and use the light ray node to show the unaltered texture to the camera and the tweaked to the scene.
you could also find a way to add more blue to the water?
manipulate environmental lighting textures same as standard material textures in Node editor (world)

you could also
limit impact of the reflections on water surface
mask elements and do some fixes after, in compositing
it all depends on what you want to manipulate and how… care to show where you’re at and what your goal is (reference)?
I am making a comics so I don’t really need photorealistic renders. So just some blue light should be enough, thanks.
I am using node trees seen on youtube for very shallow lake; the bottom is from sand/dirt, the surface is transparent so actually the blue comes from the sky.