Technicolor film was not color film, and it did not produce anything like lifelike colors. But how did it produce color movies? And why did those colors glow? And why did all of it lead to Dorothy getting ruby slippers?
As anyone who has seen an old Technicolor film knows, it looks weird. Blue eyes look like they glow. Pink faces look like they've been painted peach. Red looks scary. It all looks dyed, not recorded — and that's because it was. There wasn't any color film at the time that Technicolor was making its big splash. No one had figured out how to create a film stock that would record color. They had, however, found a way to make film stock that would filter out all the color that shouldn't get through. And they had dyes. By putting them together, they made Technicolor glorious.
Technicolor cameras didn't film in color. Instead they filmed in black and white, with different filters. The light entered the camera lens and went to a half-silvered mirror, which split the beam of light through a magenta lens (red and blue together), and a green lens. Behind the magenta lens was film sensitive to blue light, and film sensitive to red light. Behind the green lens was just plain light-sensitive film. The production crew ended up with three sets of film, one showing the blue light that the object they were filming gave off, one showing the red light, and one showing the green. All of these were black and white film strips. They just showed odd-looking pictures. For example, on the 'blue' roll, a person with few blue tones in their skin would have skin that looked completely dark, while a blue ribbon would look pure white. Each of the black and white images were dyed with its proper color. Sometimes, when they were dyed, the films would combined and re-filtered, so that only bright blues would make it out of the blue-dyed film.
At last, when it was time for the film to be shown, the three dyed films would be layered on top of one another, to make 'glorious technicolor.' This is why, especially in the early films, the coloring looks so painterly. It's also why most early films nearly cause eyestrain — especially the famous film The Wizard of Oz. The Technicolor process was expensive, not just because of the camera and the technicians needed to film, but the many incidental technical issues. Some colors, if they were too subtle, would suddenly pop out as other colors. Yellows would turn green in random patches, if too much of the wrong color was filtered out. And since the beam of light entering the camera was split between multiple films, only a fraction of the light in the studio made it onto any one film. This meant that the studio needed to be extra bright. Many regulars on The Wizard of Oz complained of eye damage from the studio lights, that lasted years.
Mostly, though, it meant that production teams didn't want to waste a single opportunity for color. In the books, Dorothy's slippers were silver, and the yellow brick road, silver slippers, and emerald green city were said to represent the gold standard, silverites, and paper money. But silver slippers were no fun at all on film. They might as well be filmed in black and white. Red, however, was easy to see, eye-catching, and didn't wreak havoc with the color mixing — the way the famed yellow-brick road often did. Hence the ruby slippers, which made the most of Technicolor, and an American fairy-tale was changed forever.