To quote the article: "He says the psychologists wrongly assumed that the monkey began by valuing all three colors equally."

So, basically psychologists are dumb; why assume that monkeys have no color preference? Even if you don't think they've got any intelligence, you could probably assume that they have some knowledge of colors associated with looking for food.

But also from the article: "After identifying three colors preferred about equally by a monkey -"

They didn't assume there were no preferences. Their mistake was to assume they could measure them precisely enough.

This monkey thinks that perceived novelty/rarity could also explain the green-preference. When at the second forced-choice, blue has appeared in 2-of-4 positions, while both red and green have only appeared in 1-of-4 positions each. Further, blue has always been at least one choice. That makes it look common, so less interesting.