Skylight is the indirect light and it contributes giving the predominant environment color to your scene. You can remove your skylight also and see what Im talking about or do the experiment I have suggested. You mentioned in your post you have a skylight at your scene too.
I would suggest for you to watch this video which is quite recent and it tells the influence of lighting in the environment. You can try replicate what the guy in the video does and it will give to you a better understanding on the subject. : Volumetric Fog and Lighting in Unre