Yeah, just like that. Now does the chat spam two large numbers, or does it spawn 0s?
It actually looks like I'll have to add a hook to a ModPlayer class to make that possible.
The easiest way would be to use a ModPlayer class, which tModLoader does not have yet. At the moment as a workaround, in the OnHit hook for the GlobalItem, you can iterate through the player's armor field from 3 to 9 and if the accessory is present then apply your debuff.
If you mean actually adding light, you can just use Lighting.AddLight in the UpdateArmorSet hook.
If you mean changing the light color the player renders in, you'll need to wait for a ModPlayer class.