Green and purple bird droppings?

I notice that there are some green and purple bird droppings outside my window. Why do they have colors?

Thanks,
Cui

Purple is likely from berries, but I do not know what could be green. Caterpillars??? Different foods effect bird droppings as they do with humans.

Or maybe the purple bleaches to green in the sun or rain? The green ones look older.

Resin