Robstyle is right. Most of the additive mixed in for cold casting goes unseen and un-noticed, which can be a waste of money when it's an expensive thing like powdered brass.
I would recommend tinting your resin with pigments to the color (black/graphite) that you want, and then dusting the inside of your mold with a good layer of the graphite powder. The resin will then have the graphite where you want it - on the surface. Then you can buff it after casting.
It sounds like a clear coat is important due to graphites softness, and tendency to come off on your hands.
I guess I should chime in here as I sort of disagree with these sentiments. I don't know what kind of props you guys are usually going for or for what purposes, but I can attest to the fact I do a lot of costuming where props require not only to look good, but be durable, cold casting for specific props is quite beneficial. CC aluminum is a good example of this, as it's likely one of the most common forms.
Having worked on Jango pieces for 95% of my entire hobby "career," nothing IMO has held up better than using CC armor/helmet/gauntlets when compared to Rub&Buff or similar paints. There's few paints (economical) that truly hold their look while also being able to last. Yea, CC can be expensive but it's also not nearly as cost prohibitive as one would need when compared to needing specialty painting equipment and space either.
My overall point is that CC is a viable option for both appearance AND durability that you can't just easily get through alternative means, and on top of it, looks the most realistic at the end of the day. Sure, if you just want a dust collector then maybe just painting it will be enough for you, but when you have props that need to be worn and handled, possibly even take a beating every now and then, it's much easier for me to whip out some brasso and polish a blemish than to have to repaint the entire thing.
I'd be interested to see what can be done with Graphite in a CC sense though. There's a few options to choose from but I don't think I've seen that used yet.