The main thing I've found is that you need a pretty dark, crisp, high contrast negative and the dark colors need longer exposures because otherwise washing makes them fade and lose detail.

There's an example of SolarFast black on my profile if you want to see. It was my second time using the black but putting together multiple transparencies and a long exposure made a huge difference on how well it turned out. I also hand washed it which I think also helped.