Calculate the relative luminance, or brightness, of a color. Accepts values 0-255 for R, G, B.

relativeLuminanceW3C.js

// from http://www.w3.org/TR/WCAG20/#relativeluminancedef
function relativeLuminanceW3C(R8bit, G8bit, B8bit) {

    var RsRGB = R8bit/255;
    var GsRGB = G8bit/255;
    var BsRGB = B8bit/255;

    var R = (RsRGB <= 0.03928) ? RsRGB/12.92 : Math.pow((RsRGB+0.055)/1.055, 2.4);
    var G = (GsRGB <= 0.03928) ? GsRGB/12.92 : Math.pow((GsRGB+0.055)/1.055, 2.4);
    var B = (BsRGB <= 0.03928) ? BsRGB/12.92 : Math.pow((BsRGB+0.055)/1.055, 2.4);

    // For the sRGB colorspace, the relative luminance of a color is defined as: 
    var L = 0.2126 * R + 0.7152 * G + 0.0722 * B;

    return L;
}

Check : http://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color/17343790
For other values