To me the numbers imply the steps are sequential, so I'd say the answer is actually turn 1

I think you and OP have a different definition of [subject verb object]...

My reading is that on line 1, no action happens, because Roy isn't yellow.

The order seems clear to me: "If a yellow gloober flomps a blue gloober then the blue gloober turns yellow,"

I still don't see how 4 could be a correct answer, could you post a plausible sequence of turns where Blue turns yellow on turn 4?

List of yellow gloobers after each turn.

1. Zoo

2. Zoo, Bat, Crystal

3. Zoo, Bat, Crystal, X-Ray

4. Zoo, Bat, Crystal, X-Ray, Blue

The text says "Every turn each gloober flomps the same gloobers."

If I understand you correctly, you are applying only one of the steps each turn, in order, which contradicts the instructions by flomping different gloobers each turn.

It is also guessable in multiple ways which is unfortunate. "There are four steps specified so I guess it happens on turn four." is one invalid way of getting the same answer.

> If gloober Zoo is the only gloober that is yellow