Because my face gradually changes color throughout the year, it’s hard to only ever use even just the same two foundation shades.
I usually buy a shade that matches me in the winter and a shade that matches me during the summer and mix them to achieve different shades throughout the year.
I’ve just resigned to the idea that I will rarely be able to use a foundation shade on its own, without mixing it with another shade.
I’ve also tried using various dark foundation mixers, and although in theory it sounds like a great idea, in practice I’ve always found the undertone to be really off.
You could try a darker powder, but in my experience powders wear off faster than foundation so you’d have to reapply. I don’t think you’d be able to get a lot of coverage from a powder foundation though so it wouldn’t work if there was a massive shade difference.