Trim paint yellows over time due to several factors. Oxidation is a primary cause, occurring when paint is exposed to air and sunlight. This chemical reaction leads to a gradual color change. Oil-based paints are particularly prone to yellowing compared to water-based alternatives. Exposure to heat and humidity can accelerate the process. Cigarette smoke and cooking fumes can also contribute to discoloration. Some cleaning products may react with the paint, causing yellowing. The quality of the paint and its pigments play a role, with higher-quality paints often resisting yellowing better. Regular cleaning and maintenance can help slow this natural aging process.