I came up with a possible solution; let me know how well it worked...I didn't want to paint everything black, but perhaps I should have done.
I have some old GW inks/washes (in hexagonal pots) including a black one. I figured that I'd just put this over most of their clothes, then make a little effort to rub it off some of the highlight parts.
The result is that they look pretty near-black, but the different colours that were there underneath help create slightly different shades of black. You can perhaps see it most clearly on the one in the middle. His cloak was blue, top purple, and trousers green. Those colours still show through a bit, at least up close, but in average tabletop conditions he'll probably appear mostly black.