There is an old song from the 1940s titled When My Blue Moon Turns to Gold Again, sung by Merle Haggard, Elvis Presley and others.  Of course, when blues start to shift their hue in a painting, it is not generally considered a good thing.  Sometimes ultramarine blue and other blues will start to turn green over time.  This is not because of a problem with the pigment, but is due to the linseed oil used in the paint.  Linseed oil slowly turns a golden yellow when it dries.  You probably have observed this if you have an old paint can in the garage with dried linseed oil around the lid.  Artist paint manufacturers know about this problem, and so they substitute other oils such as walnut, safflower, or poppy for the linseed oil in white and blue paints.  These oils dry a bit slower, but do not have such a tendency to yellow over time.  Hopefully, with well made paints, my blue moon will never turn to gold again!