Nope, not wrong. The fact is the sky isn't a set color. Point with standing, an opinion cannot be described as right and wrong. Ask a teacher(not Taboo, he just googles stuff).
An opinion is anything subjective, and subjective beliefs can be incorrect.
Here's an example:
In past history it was common knowledge among people that the earth was flat, but our modern knowledge of physics along with satellite imagery prove that the Earth is not flat. Therefore the beliefs of the past were in fact both subjective and also incorrect.
That is indisputable proof.