Item Rating(potential) and Enhancement(star) System

Let's say if you see +12%stat, that item is sure to have yellow border (because only Unique bonuses can reach that).
However, a 3% bonus doesn't mean it has to be any particular color, since lvl 71+ items can have 3% with Rare (Blue border) while a Yellow border (Unique) can also have +3% but that's for items below lvl 30.

The border color denotes the item's quality: Rare/Epic/Unique, nothing more.

Or perhaps the border is determined after the stats have been chosen. It could be possible that 1-3 stat bonuses are chosen randomly from the subset of bonuses where each bonus meets the item type and level requirement. And afterwards, the rank (colored border) of the item is determined based on the highest rank of the selected bonuses.

Another possibility is that the rank is first selected, and only bonuses up to the selected rank can be picked from. And afterwards if the highest ranked bonus is has a lower rank than the originally selected rank, the item is de-ranked.