showed how virtual mixed-color filaments could be created by alternating thin layers of differently colored materials.... The principle is well-known from traditional 2D printing. It uses Cyan, Magenta, and Yellow (CMY) inks in a method called Halftoning, which allows a printer to produce continuous tones by varying the size and spacing of small ink dots. Because of the nature of FDM printing, we don’t use a combination of dots, but we alternate the colors by layer.

... That doesn't sound like something that should have been difficult to achieve? Has it really not worked this way before in 3d printing?