Spatial color mixing (and von Bezold spreading)! This stuff's actually super fascinating and useful for designers of military camouflage! Also, the original image shows the effect way better than the overly compressed JPEG in the OP: www.psy.ritsumei.ac.jp/akitaoka/Mon...

So, what I'm curious about now, having seen this for the first time, is why the effect, at least for me, is much stronger at the center of the spiral than if I move it out to somewhere where I can only see two bands of it.