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.

Except for fading (or other weather or srorage condition that influenced the stamps' colours), I can imaging only one explanation.

Your stamps are CTOs, and in the USSR, QC rejected stamp sheets were sometimes/often cancelled to order and thus sold to collectors.

CTO stamps were never popular here among more or less seriuos colletors who care to see what they get, and not just fill places in their albums.

Therefore, such minor chages (that never occurred in mint stamps) were left unnoticed.

There were several notes in phiatelic magazines here on such finds among CTO stamps; however, nobody ever cared to undertake a real research.

No special mention in Soloviev catalogue