Although kitchen cabinets will invariably show their age, paying attention to several key discoloring culprits can help you keep them looking their best. Test any cleaning solutions on a hidden portion of the cabinets before use to ensure they don't remove paint or varnish or further discolor the cabinets.
Grease
Kitchen cabinets near the stove often suffer from grease spatters, and the buildup of dried oil can discolor the cabinet surface, giving painted cabinets a yellowish hue and darkening stained cabinets. Remove grease spatters immediately after you're finished cooking with a warm, wet cloth, and remove dried oil buildup with a solution of equal parts vinegar and water. Stubborn buildup on wood cabinets may require the use of a commercial cleaner.
Heat
Excessive heat from appliances can cause discoloration of kitchen cabinets. The high heat of a self-cleaning oven, for example, can darken the surface of the surrounding cabinetry if the oven's gasket does not seal correctly. If you notice any sign of discoloration on the cabinets after running a self-cleaning cycle, replace the gasket or have a licensed service person replace it to prevent future damage. Don't use heat-emitting countertop appliances, such as toasters, directly underneath a kitchen cabinet.
Moisture
Moisture from spills or steam can discolor kitchen cabinets. The steam from a coffeemaker, for example, can discolor a cabinet door. Place the coffeemaker well away from any exposed cabinetry before you turn it on. Immediately wipe up any countertop spills that trickle down the lower cabinets. If anything other than water is spilled, gently clean the cabinet surface with a mixture of equal parts vinegar and water before the spill dries and you risk chemical discoloration. Do not hang wet towels where they will make contact with the cabinet surface.
Touching
Simple day-to-day use can discolor kitchen cabinets. The oil from your fingertips can eventually etch away at a coating of varnish, for example, and discolor the wood underneath. You may also transfer discoloring residues such as newspaper ink or grease to the cabinets when you open them. Always use the pulls to open your kitchen cabinets, and take care not to swipe your fingertips against the surface behind or beside the pulls. Immediately remove any discoloring marks with a wet cloth.