It used the direct bleach-out method of color reproduction first proposed by Louis Ducos du Hauron and Charles Cros in the late 1860s and based on the fact that a substance can only be affected by exposure to light if it absorbs some of that light.
The Uto paper came coated with a mixture of cyan, magenta and yellow dyes which appeared nearly black.
Less strongly colored light bleached all three dyes to differing degrees, and white light bleached all three equally, leaving only a grayish dye mixture or the bare white paper.
In practice, an hours-long exposure to direct sunlight was required to completely bleach the dyes, making Utocolor paper useless in a camera.
Although rinsing out the catalyst stabilized the prints and made them much less sensitive to light, it did not actually "fix" them by removing all sensitivity, so they were unfit for prolonged open display and were best kept in the protective darkness of a closed album when not being examined.