Hi guys,
I have questions about pre-shading. What is it? How do you do it? When is this being done (i.e it does depend on the final colour of the model or it doesn't)?
Thank you in advance for your help.

I'm not a expert modeller, but may I cant help you. Pre-shade is a process done to highlight panel lines, specially in planes. It's applyed over a base paint, usually made with black or dark grey. With an airbrush you mark the panel lines with these dark color. After you ganna cover with the final color, leaving something like a shadow in the lines you wanna highlight.
The pic below shows the pre-shade over the base color:


And after the main color is applied:

Note like the lines are highlighted by the pre-shade.

Pre-shading is a painting technique based on the fact that paint fades more quickly the further it is from edge of the metal sheet to which it is applied.

Seriously, it's a concept promoted several years ago in FSM to help differentiate the panels of a natural metal aircraft. From there, it jumped virus like, to painted aircraft and armor.

Ironically, FSM spoofed it in its April issue where they present a diorama of a weathering contest for "real" aircraft.