>>Yes that's right.
>>I guess he used some kind of transparencies.
>>Even so, if he used filters, wouldn't the entire photo be green-ish or blue-ish or red-ish?
>
>Well only the green componenets would project onto the paper through a gtreejn filter - all the other colours would be masked off. So the bits of the paper that need to take the red and the blue would be shielded from any light. Then the same for the other 2 colours. But afaik IT STILL NEEDS COLOURED PHOTOGRAPHY PAPER!! So where this guy got it from I don't know ;-{

I think he printed them in black and white, the colors were for projections only. When you're projecting, you're adding colors (RGB color space); in print, however, you're subtracting colors, because it's the reflected wavelengths that count (the CMY color space). To go from RGB negatives to CMY color separation would probably be close to impossible in those times, or would at least require some heavy mumbo-jumbo with intermediate positives and yet another negative etc etc. See the link Naomi provided, it does explain this briefly.

>Bah, Alex don't know squat about anything but digital!

But the principle is the same - the color negative has RGB sensitive layers. The CCD chip has RGB sensitive elements. Projecting colors on your screen or via your slide projector works in pretty much the same way, adding elementary color grain to build any shade of any color.