Black primer is good, grey and white aren't too well suited to this method. Vallejo model colour (VMC) burnt umber -> drybrush VMC beige brown -> a thin, dark brown wash all over - painted very thinly on the brush so as not to pool, and a VMC black+dark brown wash between ribs, eyes, mouth etc -> beige brown + dark sand (or VMC Buff) -> highlights with dark sand or buff again, its slightly yellower and darker, for a more worn look.) Pick out teeth with VMC Pale sand, though this isn't critical. I usually drybrush all the layers to make them look pitted and craggy, but sometimes overbrush (wetter paint and side of the brush hairs) for a cleaner, sharper look.
Then comes the fun part - matte glazing medium + paint + water, equal parts of each. Paint more or less depending on where the skellies came from and how dirty you want them. ie. desert skellies will have only a very little rusty or sandy brown colour, dab on with a brush then dab off with some paper - should leave a very faint stain/mottling here and there. Normal graveyard skellies will get some brown and a touch of green dabbed on with an old brush with split ends. Skellies from damp areas will get more green than brown, with a gloss medium green mix added after varnishing to give it that wet algae look!
Also, for the mods, I think this thread shouldn't be in the Challenges section?
