If Theus had meant the witches to be colorful, He wouldn't have made them so damn scary. Stay away from the huge side-pannier skirts of Louis XIV, though. Once again, I have to remind myself that this is just a game, so one shouldn't neccesarily exhaust oneself as if prepping for a Rennaisance Faire. There is no precise 'period' that has to be adhered to, so that gives you a lot of flexibility for inspirations and patterns. A viable option is mix and match, especially if you've got some ability to sew.
Skirts are easy to make and there are lots of patterns out there.
Try laying black burnout velvet over burgandy satin, or black lace over a black solid. Or even just yards and yards of a subtly patterned black satin. That leaves the tricky part - what to wear on your top half. They're a little flashy for a Fate Witch, maybe, but everyone will recognize you're putting in a credible effort. Pattern Reccomendations Below are some patterns that I have found and evaluated.