From an artistic standpoint, black is theoretically the combination of all colors, with white being the absence of any color. From a scientific standpoint it is exactly the opposite, with white being the presence of all color and black being the absence of any color. So depending on who you ask, a scientist or an artist, you will get different answers on what black and white really are.
Since you are using the scientific standpoint for your argument on white and black Autumn Moon , I'd challenge you to think for a moment about the other colors. Scientifically speaking, the color green is not green at all, it's actually the presence of every color excluding the color that you are seeing. (The green light is being reflected, which then hits your eyes and tells your brain the candle is green, all the other colors are absorbed by the object.) I guess that would open up a whole new range of questions, perhaps that you can think about for later. :)
I suppose it all comes down to what you believe, personally I associate white with purity due to a combination of reasoning I'm not sure I can adequately explain. Guess it all depends on your definition of purity and what the colors each mean to you.