In particular, if you're going to reflect that dark purple color in his outfit, what are you going to use? Is there a particular Citadel paint worth picking up?

My plan is to use a dark grey-ish basecoat with a wash of Druchii Violet from GW to get the feint purple tint of his robes.

You can cover this from multiple angles. Anakins Robes are not straight up black, and they are mostly leather which as quit some sheen to it. You can resemble this if you mix some black with some brown and add some purple if you want to. This will give the robe a nice base coat and you have some Purple in your shadows. You can highlight this mix by either mixing in a bit of lighter grey of brown depending on where you want to go with your look.
Extremly dark Colors can be tricky to highlight, so use caution when lightening up you mix. And don't paint pure black as a base coat. It will look boring and leaves you less room to go to with your highlight regarding color

I know this is of course a different outfit, but I used a 1:1 mix of Mephiston Red and Rhinox Hide (both GW) for his red/purpleish areas. (I washed it with 1:1 of Nuln:Lahmain medium).

Cool. Thinking of picking up some dark gray base then, + Druchii Violet. I guess a different brown wouldn't hurt, but I guess I can get by lighening up the darker citadel brown that I have.

I guess you mean the reflections of his saber. Maybe scale 75 carribean blue and vallejo's blue violet to tint it.

3 hours ago, Billion Year Old Carbon said:

Cool. Thinking of picking up some dark gray base then, + Druchii Violet. I guess a different brown wouldn't hurt, but I guess I can get by lighening up the darker citadel brown that I have.

Ha, I ended up picking up Rhinox hide due to getting fed up of having to keep darkening a lighter brown.

P.s. Don't just use white to lighten, a lighter tone like sand/flesh etc works better, as it doesn't dull the tone down so much 🤘