Ah curiousity got the better of me:

To acquire a color NTSC signal from a Quadra (or any Apple Macintosh display card), an RGB-to-NTSC converter is required, such as those available from RasterOps, Truevision, and Computer Friends.

Click to expand...

So it looks as though the Apple Presentation System is your best bet for colour, unless you feel like doing more research and building something. That something will be more than just a few diodes and wires though. I have no doubt that a bit of Googling would turn up a home build RGB-NTSC converter project, maybe even a kit or two.