Hold up, I’m confused. How could people who didn’t know Russian read them at all? There’s probably something obvious I’m missing, but I just can’t parse this at all.
So you might have, in a red font, the word "blue", followed by the word "green" in yellow letters. The correct response is to say out loud "red, yellow". This is hard to do when the words are in your native language. So if the words are in Russian, it's an easier task for someone who doesn't read that language, because they aren't distracted by the meaning of the word and can just see the colors.
See the example on the wiki page for reference: https://en.wikipedia.org/wiki/Stroop_effect
Imagine красный written in Blue
With the Russian word you just see a funny shape and it's blue.
With the English word you see red and blue at the same time!