I have a very delicate white paper hat that has turned yellow on several areas. How can I whiten it? Can I bleach it? Send it to the dry clean? ...Paint it? I have considered using soapy water but I'm afraid it wouldn't be gentle enough.
Liquids might damage it. The following suggestions are from mildest to more likely to damage the hat:
Exposure to sunlight (or UV lamp). This is less predictable in removing color.
Gently spraying with with water and then exposing the hat to chlorine gas. Unless one is familiar with safety precautions for handling toxic chemicals, leave this to a professional. Spraying bleach (e.g. sodium hypochlorite) directly on the hat might remove the stain, but the dried residue could cause the paper to disintegrate later.