They were black and whites taken with color filters over the lens (R, G, B).
Project each of the three in its corresponding color and overlap them to produce a full-color photo. Overlap them in that color in Photoshop to produce the digital images you see here.
How would you project them in their corresponding colors, though? I'd imagine you would have to produce prints in the colors. Only other thing I can think of is to project the negatives with R/G/B colored light behind them and make those projections line up.
Project each of the three in its corresponding color and overlap them to produce a full-color photo. Overlap them in that color in Photoshop to produce the digital images you see here.