I don't think there's one best way but the safest way is to mix the finished colour* and apply that, rather than attempting to do the effect with a thin coating of black or another dark colour.
But if you're working in oils and you do go a little heavy with a dark colour you can always wipe off and start again if you had to, or gently stipple off the excess with a clean, dry, brush. In fact that's one method to doing this sort of thing - paint on a thick coat and then remove a lot of it (same basic technique can be used to do worn leather, stains, weathering on armour).
*If you want to try this it is often hard to see the colour that will do the trick correctly so one method I like to suggest is to mix the greyish colour you think you need on the palette but then blend a small amount of it about 1:3 with your basic flesh colour, now load a brush with that and hold it next to the face - often you'll find even diluted that much it's still darker than it should be!
Einion