When comparing items to equipped items, is the red how much will be lost if equipped or does it mean that there is nothing added to that particular attribute?

How much would be lost.

When comparing items to equipped items, is the red how much will be lost if equipped or does it mean that there is nothing added to that particular attribute?

What Beemerchick said. Overall:

* Red = the previewed item is worse in that stat than the equipped item.

* Green = the previewed item is better in that stat than the equipped item.

* Stat not listed = no difference OR neither item has the stat (which is also "no difference", since zero and zero aren't different).

* All red = probably you're comparing a lowbie item with a highbie item, but it might also be because you're comparing an empty moddable item with a properly set up item.