Don’t worry! This is completely natural! Discolouration of foam is not harmful or toxic in any way!Foam turns yellow mainly due to oxidation. Things like UV light, high temperatures, air pollutants, sweat, body oils, and more are all responsible for the oxidation process. While there’s no way to remove the yellow discolouration, there are ways to help slow the oxidation process.Use the water resistant liners. This helps avoid sweat and body oils and any other spills or mishaps from staining the foam.Avoid direct sunlight, even if the water resistant liners are still on. When washing the covers, try to keep the foam away from windows and direct sun.Try to keep the foam away from heat sources. This includes windows (sunlight through the glass creates heat), heating vents, fireplaces, etc. Heat will speed up the oxidation process! Bottom line, the yellow discolouration will not affect the use of your foam in any way, nor is it harmful or unsafe for your little ones to play on!