It shows that yellow color when you use a Potion of Mastery(exotic Potion of Strength) in the equipment.

Its strength requirements

That battleaxe should have a base of 16 strength to use it.when it's upgraded to +1 it should require 15. The fact that it's at 13 means that you used another item to influence it's strength requirement which should be the upgraded strength potion that lowers strength requirements for a single item by 2. That's why it's yellow probably

I didn't even think about the potion! Thank you so much