I have been doing this since i was a kid about 12 years old and not really understanding. When i focus on something intently the object in question(especially if it emanates light) will become the only colored object in my field of vision and everything else will become grey and white and very dark almost pitch black sometimes. Sometimes when i would look at the light reflecting off the fine sweat on my skin intently the color of my skin will turn purple but i don't think it's an aura because auras are supposed to look like mist around your body. Does anyone know what this is? my explanation is that humans have more color sensing cones in one patch of their eyes (favola?) and i use it primarily when i am using to focus on the object, and the majority of hte rest are rods which then make up the rest of my feild of vision. Now this does not explain why some objects attain strange colors and why sometimes the rest of my feild of vision goes almost black. also this works best if i do not blink for 20 seconds atleast, and i have full control of it, but just don't know what it is, if it's useful or just a curiosity of the human eye. hope somebody knows something about this! 
