Sometimes I do not paint the outside, just polish
if the plastic is of good finish,
no sink marks or "weld lines" from the material flow.
This to avoid problems with dust and brushmarks
In those cases I paint the inside in the approximately same colour as the outside ,
this to avoid the "shine through" discusses above.
Particularlily neccessary on bonnets with thin sections for making cut outs for hood scoops.
Also the bonnets can have major variations in material thickness
due to the inner structure.
If one paints the inside dark grey or black to emulate either painting or sound dampening
that will be visible from the outside
if not painted as example white if the exterior is planned to be white.
The same goes for the roof:
if the inner headliner is supposed to be dark ,
first paint a couple of layers of the exterior color
before applying the dark colour.
What is then gained by this:
You get a tidy model exteriorwise,
with a not as high gloss as with paint but which can be of quite realistic gloss,
i.e for older cars , especially race cars.
The painting of the inside does not require the same wotk on the finish
as on the exterior = is simpler.
Laziness ? Perhaps ....
On cars with decals on I put a layer of clear.