So Valejo air + masking tape, you get that, but that's only the beginning of the story
Here are the colours I used to paint the engine, and the mostly the same for the fig and the groundwork, ity's important to show this because notice the use of the blue and red. Notice that the colours are actually blended together. Never underestimate the importance of actually COLOURING the scene!
Then I overpaint the metal sheet using the usual wet on wet technique, blending paint with pigments (Okay, MIG pigments..).
One good thing, rub the pigments covered surface and those just stick in the crevices, you can tone down the effects with wetting and putting a bit of paint later.