Board Thread:Dark Souls II/@comment-184.56.17.23-20140315092343/@comment-5264386-20140315121158

It's going red to indicate that you have more weight equipped, meaning that, in terms of weight, it's worse than your other equipped item. The inverse is true for when text goes blue; it's saying that the item is light, and thus better weight-wise. Also, the Heide Knight Sword has 75 Physical Attack and 50 Lightning Attack, so that's why it appears to be weaker.