Instruments finished in nitrocellulose lacquer not only tend to craze with greater ease than the newest catalyzed finishes but they also yellow with age. Instruments normally consist of a color coat and clear coat, with the exception of a natural finish which is just clear coated. This top clear coat will yellow with age. This can turn a white guitar yellow and, since blue and yellow make green...older blue instruments can eventually take on a much more green appearance. This phenomenon is a simple fact of life. While it's true that UV rays may accelerate the process (just peak under a pickguard), nothing can be done to stop nitrocellulose from yellowing. In fact, like crazing, it is a common and expected occurrence.