I'd suggest that they either did use a darker shade, or they did the following:
Undercoat first. It's the thing that makes painting upon different bases to get the desired results possible, they flat out lied at botcon. But I suppose it isn't a lie when they only work on the one principle of a thin layer of paint with no preparation before hand.
Multiple layers.
Just thicker application of paint.
Also about classics bumblebee, it just happened to be the figure that I first noticed the poor paint applications starting to appear on. Mine and others that I saw at the store all had fade through on the roof due to cheap skating the paint application. Sure it wasn't as noticable at first glance like sunstreaker is, but it's there, and when you're as particular about the appearance of the finish of toys as I am, you notice.