Fidenza uses probabilistic color palettes. These are a set of colors that are selected by hand to work well together. Each color in the palette is assigned a probability. So, for example, you can say that about 50% of the shapes will be blue, 25% will be red, 10% yellow, and so on. For Fidenza, I prepared 14 different probabilistic color palettes, one of which uses a different probability technique.
-Tyler Hobbs