The tricky part is knowing that white at opacity 1 will become the target color, so variations from that are all done with changes of opacity to let the background darken (or lighten if set to a darker color and on a light background). Obviously they can’t be more than monochrome, like having a white piece of paper inside while the rest is manila, in order for this recoloring to work. We could instead just have one unchangeable full-color folder icon, but that could get tricky on multiple backgrounds and I like the idea of having manila folders on Windows or Blue folders on Mac if we want.