If you are still getting blacks that turn gray in shadows, then your black is too dark. I work with black a ton because of the German crosses on the wings. I've settled on black paint being only 85% black. It never looks off in shadows.
Let's look at my skin in the GIMP window. I also have the color selection window open so you can see where to plug in RGB numbers.
The night fighter color is the selected color in the window. You can see all the elements that make up this specific shade. It is represented by positions on 6 sliders. H is "hue", S is "saturation" and V is "value". The bottom 3 make up the RGB - red, green, and blue. Just plug in a given RGB number set into those areas and the top 3 sliders will adjust accordingly. Now pay attention to the position of the V slider. That slider moves from 0(Black) to 100(White) and can indicate just how bright a given color is.
So RLM 22 is RGB 42/41/46 and the V for that RGB is 18 - meaning 18% white
The black I use - found in the shield emblem - is RGB 38/38/38 with a V of 15.
The black in the cannon ejection chute is RGB 26/26/26 with a V of 10.
Pure black is RGB 0/0/0 and the V is also 0
It's not the Hue that eliminates the gray-out of black, but the Value. You do not want to go all black, just mostly black.
