There's sometime an o with a bar blue or yellow on it...what does it mean?
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oh ok thanks and also wanted to know why the change of colors? is it for it's rarety?
There are some differences between them, but they ultimately mean the same thing.
Blue is just the general "cannot be sold or traded", yellow is "this item has become account bound because it was used" (this is primarily for weapons and units and in some cases it can be removed with an owner reset pass), and red is "permanently account bound" (you mostly only see this when using the weapon transmutation system).