Summary: Technicolor filmed simultaneously on three B&W films after passing through color filters. Each film strip was dyed red, green, blue, and the three were overlaid for the final film. Set lighting had to be extra bright. The ruby slippers were actually silver in the books.
http://www.loc.gov/pictures/search/?q=color&st=grid&co=prok
Kind of a fun fact is that in a sense we use this same idea today for digital photography - modern camera sensors are almost entirely[0] monochrome, so we put a tiled color filter in front of the sensor, which makes each pixel pick up a different color (like the 3 colors of the film strip), and then we digitally reconstruct full color at all pixels from the individual colored-monochrome pixels. So it's basically just a 3-strip image, but shot on the same image and digitally reconstructed.
https://en.wikipedia.org/wiki/Color_filter_array
https://en.wikipedia.org/wiki/Bayer_filter
[0] the exception is Foveon sensors which actually do have three sensors at each "pixel", stacked on top of each other. The problem is this generally has lower resolution than an equivalent Bayer filter-based sensor - however somewhat offset by a relative lack of moire due to the higher color resolution. Most (but not all) Bayer sensor cameras use an antialiasing filter (optical low pass filter) over the sensor to combat this - which costs some resolution (eg Sony A7 vs A7r - the base model has the moire filter and the R doesn't, but the R model gets almost twice as much resolution).
If you have a knife, you can scrape off the color layers in order. Starting from the emulsion side (the side which is less shiny), you'll scrape of the magenta layer first, then the yellow, and you'll be left with cyan on the bottom.
It's always in this order, due to the physics of how film works.
Although fun fact some of the early Minolta digital SLRs (“RD-100”?) actually did have three separate sensors, and also interestingly they used telecompressors to “resize” the image down to the smaller sensor so that the lenses effective focal length didn’t get super long on the tiny sensors they could cost-effectively produce in 1993. And this also somewhat mitigated the loss of light from having to split the light across multiple sensors.
Interpreted this way, the yellow brick road is the gold standard, the silver shoes are the Silverite 16-to-1 exchange ratio of silver to gold, and Oz? Abbreviation for ounce.
Evidence that this was Baum's intention is... thin, but I've always liked it. For the curious: https://en.wikipedia.org/wiki/Political_interpretations_of_T...
[0] http://www.digital-intermediate.co.uk/examples/3strip/techni...
[1] https://it.wikipedia.org/wiki/Technicolor#/media/File:Techni...