This is how I generally go about creating a panel when I draw it myself.
Billy the Cat © D.C. Thomson & Co. Ltd.

1. The foreground linework is place on a top layer [normal mode, with a transparent background]:

2. Colour is applied on layers below the linework layer [first as flat colours, then rendered with high- and lowlights]. A flare is placed on a layer above the linework, glinting off Billy's helmet:

3. A second layer of linework is added beneath these layers [with colour applied similarly]:

4. A third linework layer is placed behind these [in this case, the linework itself is partially coloured with a green graduation for the grass]:

5. A final linework layer (i.e. 4 layers of linework altogether) is placed at the back [background colour is placed behind this and the linework has been coloured red]:

6. A screen effect layer is placed above the back two linework layers [creating depth and atmosphere]:

7. A second graduated layer in screen mode is placed below the foreground artwork [this recedes the background elements a bit more in order to focus attention on the foreground figures]: