My YF-31 Siegfried ,VF-0, and small part of my YF- 29 Alto both yellowed on me. So I bought some Cream Peroxide Developer. Applied it to both Valks. Placed them in an air tight container and let them both sit under a hot lamp over night. After 2 applications they were both white again. Word to the wise. Don't use a hot light bulb in your display case, it will yellow your Valks.