Which cell organelle is more important in giving colour to petals?
When I read your question (headline), my first thought was: "Vacuoles!"

After reading in more detail about your problem, I agree with you that there is some controversy with this question. The problem is in my opinion that the question is not well formulated. The term "more important" is not really scientific, because what decides importance? Do they mean with importance that it plays a role in more flower species? Or do they mean that the color palette is larger with vacuoles compared to chromoplasts?

I do agree with the answer though, the pigments in the vacuoles are responsible for more colors in petals, and probably also in more species. However, there might be some plants in which the petal colors are dominated by the chromoplast colors. So the question is not well phrased, "more important" is too vague for exact science.