I’ll start to paint in the details.
 Details on the characters and the foreground
I make a new layer called “details” at the top, and paint some more where I think the illustration is lacking.
In the below example, I added highlights to the eyes and changed the eyebrows and the shading of the hair.
For parts where the color is very light, I add color with a [Multiply] layer.
I paint while looking at the overall color balance.
I add color to light parts with [Multiply] layers and brighten dark parts with [Overlay] layers. When I’m happy with the color as it is, I add details on a layer set to [Normal].
 Adjustments to the background
I create a new [Overlay] layer and paint the light from the windows in the background, as well as where I want to brighten the color.
I then create a [Screen] layer and paint over the darker parts of the background in a light blue to create a natural effect. This is based on the concept of aerial perspective.
 Adjusting the color of the base painting
I don’t like the black parts from the base painting, so I create an [Overlay] layer above the base painting and paint over it in a blue color.
I reduce the opacity of the [Overlay] layer to 27% to get a good depth of color.
The illustration looks like this after I edit the color of the base painting.