There's a couple of threads highlighted in the Sticky Thread on this issue, might be good for you.
I did a 40k assassin model years ago as an exercise in black. What with the skin, the straps, and the weapons, all gonna be black I wanted to get some variety in those areas, but still all look black.
I was playing with the idea of psychology, of how we see and regard colour. We instinctively make assumptions about the texture of any given area, based on how high-low contrast it is, and how natural/organic-contrived/produced the colours used imply, or at least we infer that from them.
For the weapon casings I went with grey highlights. It's too clean and precise to be natural, so it looks artificial, a man-made plastic.
The straps and boots, wanted to convey a leather. Leather often has a blue tint applied to it in dramatic photographs, and is part of the colour palette as seen in movies like Underworld and Batman. A bit of a rich, dark blue worked in to the middle gradients of dark to light evokes that. Mostly though, it has to be high contrast to give the impression of a leather. At least, a leather that hasn't been rubbed down to a regular flat material finish.
The fancy skin suit, I went with turquoise. The idea being that we don't identify anything at dark turquoise. Try and think of things that are and I'm sure we'd all come up short. There's one though, faded tattoos. Which kinda works to suggest the rule. We see them and don't think of the as dark turquoise, just as faded tattoos because we don't have a mental stock of visual resources to make assumptions about that range of colours. We see a dark turquoise, and are simply informed by what we're seeing, and as what we're seeing in the case of an assassin mini is we know to be a black clad person we see the skin-suit as a black and take the introduction of turquoise into all that as natural.
Back then I worked turqoise into almost the whole process, from the first highlight of black-turquoise, through the dark grey-turquoise and light-grey turquoise and then let off, allowing the ensuing white to dominate the upper stages.
I've since picked up a P3 Coal Black paint and consider it a lucky stumble upon a useful paint, as it's essentially a dark turquoise, it reduces so much effort from mixing up paints. I can use that and just add white.
That mini didn't have any regular material to play with. Fantasy mini's were more helpful for that.
Like DR said, can add some Bleached Bone into the mix. Though any paint in that general area can work too, like say Bubonic Brown.
It does make for a soft range of tones. With the smidgeon on that kind of colour in there it works to suggest a natural cloth too.
It's well worth keeping the black ink to hand though. One can work away and not realise that the highlighting has taken too much black away. Can work some black back into it with a glaze. Handily, glazes seem to bind the stages of the painting process together too.
