Eldar Guardian with only it's base coat. I was experimenting whether to use Black or White for this range of colours that I am employing. After a few runs, I have concluded that White will be more appropriate as I took too much time trying to get the Warlock Purple to become visible on the armour plates. The reason I took Black in the beginning was because the Guardian's undersuit is Black. In hindsight, I should have used a White undercoat.
Use White undercoats when the Base colours cannot show through.