Aging paint can develop a yellow cast, most noticeably in the dried films of white paints or clear varnishes.

What Causes It?
One cause of yellowing is the oxidation of alkyd, oil-based paint or varnish. Heat from stoves, radiators, and heating ducts can also lead to this condition.
Finally, a lack of light (e.g., behind pictures or appliances, inside closets, etc.) may lead to yellowing.

How to Solve It
Top-quality latex paints do not tend to yellow, nor does non-yellowing varnish. Alkyd paints, because of their curing mechanism, do not tend to yellow, particularly in areas that are protected from sunlight.

Note: Images provided by The Rohm & Hass Paint Quality Institute.