You can argue an argument. Just because you call in an opinion doesn't make it one. Arguments can be judged as valid or invalid, better or worse, even right or wrong. Opinions cannot be valid or invalid, right or wrong. They could potentially be better or worse, but usually not objectively better or worse.
If you said that the color blue makes you feel happy that would be a statement of fact.
If you said you like the color blue because it makes you feel happy that would be an opinion.
If you said you should wear the color blue because it brings out the blue in your eyes that would be an argument.
If you said that the color blue
represents happiness that would be an assertion unless you have some valid reason for thinking so. In order to represent something it has to be consistently viewed that way by others. In fact typically the color blue is said to represent trust.
No, you're thinking of an assertion. Opinions are almost exclusively feelings. Now if you gave a reason for why you think this then you'd be making an argument. We can argue an argument, but not an opinion.
That's precisely what makes it an opinion, and why it's incredibly stupid to argue over pure opinions. Now it's possible to have an opinion on the quality of an argument, but the argument itself is not the opinion.
If I said I think it's going to rain because I see a dark cloud in the sky that's moving towards us.
Then another guy said, I don't think it's going to rain because the radar on my phone isn't showing any red near us.
These are two logical arguments. Both are valid. One conclusion will be proven right, the other wrong. Now in the moment you might have an opinion about which of our conclusions makes the most sense to you, but that's more of feeling you have. One of us might be able to improve our arguments to better sway your opinion, but what we're doing is not offering opinions, but in fact arguing. It is the arguments that can be better or worse, valid or invalid, not the opinion.
Here is a good reference on the difference between an opinion and an argument...
http://skillsforlearning.leedsbeckett.ac.uk/preview/content/critical_thinking/02.shtml
The key is whether you provide reasons to support your opinion. We can argue over whether the reasons you give truly support your opinion, but if all you provide is an opinion then it is foolish to argue over it.