just a related tidbit (i'll leave why they used black to others)...
the color of your primer subtly affects the depth and final color of the topcoat although this is more apparent on a larger object such as a car but the principle is the same (to my eyes anyhow. i've done alot of painting in my time).
experiment over the years and you will see....
no, you don't necessarily need to use black but some are able to achieve a certain scale "shading" effect.
by the way, a lot of bright red cars are painted yellow first to make the red "pop". want a darker deeper red? well then you know what primer to use....
looking for a toylike looking cheesy white? spray white topcoat over white primer.
going for a slightly cartoony look? paint dark shadows in before you topcoat.
experiment with paint viscosities.
and it's no more bother to paint on black primer than grey primer.
