Except the actual visible light spectrum is not a circle... That chart reflects what our brain sees, and even on that chart, there is no specific frequency for purple. All the color Except Purple, on your chart, have a frequency. Red and blue are at opposite ends of the spectrum. We can disuss RGB versus CMY, additive versus subtractive color, but when we are talking visible light, there is no purple. While yellow and blue form green, green is a color. Purple is a color that is a contruct of our eyes and brain limitations. When you mix blue and green, you get another frequency, but not purple. there are a lot of write ups on this, outside of the internet too....
I never said gray is a color, but if you are color blind, gray is the color....same as purple.
I originally brouth this up because of a discussion of color blindness. If you are color blind, either the eyes have a defect, or the wiring is messed up, or can be some other problem. color is a perception, and even if you are not color blind, pruple is a valid color to our brains (not our eyes) as gray is to someone who is color blind. Heck, with the right hallucinogens, color can became a physicality to your perception....We cannot see past violet, and on your chart, violet goes into purple....that is great for mixing paint, but does not explain why we see purple.
I end this line of posting with this You tube explaining this.
http://youtu.be/iPPYGJjKVco