perceived brightness of a color

// From http://www.nbdtech.com/Blog/archive/2008/04/27/Calculating-the-Perceived-Brightness-of-a-Color.aspx
// returns a number in the rage of 0 (black) to 255 (White) and to set the foreground color based on the Brightness method
// c is an array where 0 is red, 1 is green, 2 is blue
//rgb values are on a 0 - 255 scale

var colorBrightness = function(c){
	return Math.sqrt(
	      c[0] * c[0] * .241 + 
	      c[1] * c[1] * .691 + 
	      c[2] * c[2] * .068);
};