color model
http://en.wikipedia.org/wiki/HSL_and_HSV
http://www.easyrgb.com/math.html
http://www.easyrgb.com/index.php?X=MATH&H=20#text20
http://www.easyrgb.com/index.php?X=MATH&H=21#text21
http://mjijackson.com/2008/02/rgb-to-hsl-and-rgb-to-hsv-color-model-conversion-algorithms-in-javascript
http://en.wikipedia.org/wiki/HSL_color_space
http://en.wikipedia.org/wiki/HSV_color_space
