var/obj/item/shirt/npc_shirt
world/New()
..()
npc_shirt = new /obj/item/shirt(null)
var/icon/i = npc_shirt.icon
i += rgb(175,0,0)
npc_shirt.icon = i
...and then in my mob/npc/New() I have...
mob/npc/New()
..()
src.overlays += npc_shirt
However, this only works if I put a spawn() delay before adding the shirt overlay because the problem is that the NPC is getting the overlay BEFORE the npc_shirt object is created. What I want to do is make sure the shirt object is fully created before my NPC object is created :\