Quick edit, I'm not that good, just to show you the light was not coherent inside the room (the wall especially was too bright). Not that my edit comes with good lightning, but at least inside is darker than outside.
Try to use less colors. Less noise like people said.
Don't hesitate to use hue shifting (if you choose 3 greens colors for your tree, bring the darker color to blue, and the lighter one to yellow for example), it helps to make your pixel art look less blend.
Don't hesitate to make the outline go a bit inside the sprite if it helps separate elements like head and torso.