The toning down is best achieved by airbrushing a lighter shade of your base colour onto raised and central portions of any flat(ish) plate found on your model. For instance if you have a square panel, lightly airbrush the lighter tone in the centre of the panel, then slowly pull out of the model so that some paint lands between the centre and the edges, creating a blended look.
Then do a wash and apply it. I rarely use pure black for washes, but instead a 'dirty black' colour called 'Sepia'. It has a bit of brown in it and therefore gives the model an earthier look than pure black... Make sure the wash is not going to damage the paint below! I use enamel and acrylics for my base colours then artist's watercolours for my washes as they are very practical and 'flexible' in case I make mistakes or put too much.
Finaly, a drybrushing with yet lighter shades of the base paint allows any raised detail to 'pop out'.