To unlock the You've Got Red on You trophy in Days Gone, you will need to collect 541 items from corpses.
Items are only collected from dead human corpses, not Freakers, and since there are significantly less human enemies in the game, you'll want to
make sure you are constantly looting dead human corpses from the start of the game. If you loot multiple items from one corpse, it counts all of the individual items, which makes this a little bit easier since most of the corpses will drop a few items.
You can find human enemies in story missions, encampments and ambush camps. They can also be found in the open-world, although they are rare to find.
Missing this by not looting corpses or by having a full inventory which stops you looting will mean you have to grind at the end of the game in the search for human spawns.
You loot corpses by approaching them and pressing
.