I agree with this.
I like to manually pick shadow and light colours because slight shifts one way or the other will dramatically change the atmosphere, and can say very different things about the same picture. It also helps give you a good amount of contrast between your background and characters. They can both be bold but you can do small things like highlight your characters a tiny bit more saturated to draw focus to them. I guess it's also another element of composition that will go into the entire page when you consider lighting panel by panel.
Contrast is probably the single best strength of cel-shading, you're creating an illusion of 3D images with a very limited range of colours. This means the "jump" in value needs to be more significant between each one compared to a painter who blends.Therefore generally you don't want to start too light, or you'll wash out your light source. @Akitsukino it also strikes me that where you're placing shadows on figures is a little timid, like you're scared to give them strong casts over facial features which leads to missing shadows or otherwise strange light sources.
Eyeballs, inner corner of the eye, under the nose, inner ear, under the neck and under the bottom lip are natural crevices on the face that catch cast shadows under most lighting conditions. Just those alone can usually draw attention to a face, don't forget these important spaces.