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Raw linseed oil in its natural state is a dark amber to greenish-yellow color. Placed in a clear glass jar in a window that gets a lot of sun, it will bleach to a light straw color inside six months. Linseed oil never quite becomes water-clear, which accounts in part for the widespread use of safflower oil as a vehicle by paint makers whose customers presumeably balk at whites that are not "pure" or blues slightly "turned" by the color of a vehicle that's not water-clear. However, linseed oil produces paint films unquestionably stronger and more flexible than those of any other drying oil, bar none.
A painting made with sun-bleached linseed oil and well cared for will not darken or yellow appreciably after several decades. However, if the painting is stored in the dark, it may darken and yellow within a fairly short time (humidity exacerbates the change). It will soon return to its original state when returned to display in ample ambient sunlight.
This bleaching/darkening cycle is a normal aspect of oil paintings. Eventually, at some point several decades into the aging of the paint films the reversability of such yellowing ceases as a consequence of ongoing chemical reactions, and aged films become more transparent, as well as "yellower" with advancing age. In short, it is inevitable that oil paintings yellow as they age, but marked changes pretty much occur when the painter and his patrons have long ceased caring . . .
If ultimate permanence is the painter's goal, oil paint is not actually the very best choice. Watercolors remain unchanged by aging. Encaustic, egg tempera and buon fresco also have a much better record for unchanging permanence.
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