Russia - 1966 Color Variations

Hi - new member here. I have been going through and cataloging my very random collection of world stamps, and I came across these 1966 Russian stamps (Scott #s 3210-3212).

I'm intrigued, as they appear to have very distinct color variations, that don't appear to be simply fading. On the 4k stamp, the runner changes from a brown color to an olive green color. And the 6k stamp is listed as orange, but my second copy is clearly a yellow/ochre-ish color. I couldn't find any information on color variations within this set - does anyone have any additional info? Thank you in advance.