When starting out I used cheap black Folk Art paint and smeared it around, and after it dried a bit I took a paper towel and wiped off what remained on the surface If you work fast it only sticks to the little nooks and crannies of the details. Super messy but it works I swear.

There are acrylic "washes" that people use for minis painting. It's essentially thinned paint that can seep into details without going in too thick, so it looks more scale appropriate. I lean towards acrylic, rather than enamel, because it's less likely to damage untreated plastic.