Somewhat disappointingly I've discovered that the "Blue Black" that I was going to use to lighten the "Camo Black" seems to eat into the plastic through a primer coat, even through a primer coat & Camo Black coat to give an acid etched appearance. I don't understand why because all the other external colours, including the Camo Black, are SMS colours &
don't do that.

Even a 20% Blue Black/80% Camo Black mix eats straight through. 40% BB / 60% CB gives the tone I was looking for but, of course, eats the plastic.

I'll see how Tamiya NATO Black goes over the SMS paints (acrylic over lacquer can be problematic) because it's the closest colour to what I want but, if that doesn't work, I'll have to use the SMS Camo Black & hope that a grey oil wash lightens it a bit.

Glad I was using a test piece & didn't put it straight onto the build!
