An opinion by definition is also someone's beliefs and thoughts. If it is really their thought/belief that the sky is red, then it is their opinion as well, and their thought/belief/opinion/etc. is wrong.

"Opinions can't be wrong" just plain doesn't work due to the fact that opposite opinions exist.

"(I think) the sky is red."
"(I think) the sky is not red."

Saying "opinions can't be wrong" and that neither of these "opinions" can be wrong, is incredibly contradictory and doesn't work. One of these has to be wrong, and if one opinion is right, then the opposite opinion has to be wrong and vice versa.

"This band is bad" vs. "This band is not bad"

This works the same way. Both of these opinions can't be not wrong. One of them has to be wrong.

How can you tell someone that the sky is not red? If someone truly believes that the sky is red, you can not disprove it because you can not see what they see and you can not be inside of their brain.
oh god



If you're going to argue that "opinions can't be wrong" because they technically can't be proven wrong, you might as well just throw out the terms "right" and "wrong" from the dictionary, because nothing can be "proven" to be right or wrong by what you're saying.