It's impossible to judge "darkened by sunlight" on a sample of one looking at only one side of the chip. When color variations are caused by age, you will likely see a range of shades in a stack of chips, and variations from one side of a chip to another. Particularly with sunlight fading or darkening, the two sides will not be affected evenly. If you can take a quantity of chips and sort them by color, and each chip is the same on both sides, the variations are most likely manufacturing variations.