It’s called black basing or pre-shading. It’s not so hard once you get the hang of it. The current fashion trend is to “marble” the white on, to get even more color variation. This works especially well over large flat surfaces.
The first time I tried it on a Gundam kit, my shading was too timid and the final color was not thinned enough. I ended up with this:
The second try got considerably better results:
I have applied pre-shading to other types of models as well:
You just gotta thin the white paint a lot, use low air pressure, and go slow. I get impatient when doing a model, as it takes more time than the actual color application.
Modulation is painting different areas with slightly different shades of the same color. Basically the intent is to emulate how natural light falls in uneven intensities over a surface.