getRelativeLuminance() {
        const R = Color._relativeLuminanceForComponent(this.rgba.r);
        const G = Color._relativeLuminanceForComponent(this.rgba.g);
        const B = Color._relativeLuminanceForComponent(this.rgba.b);
        const luminance = 0.2126 * R + 0.7152 * G + 0.0722 * B;
        return roundFloat(luminance, 4);
    }
http://www.w3.org/TR/WCAG20/#relativeluminancedef
Returns the number in the set [0, 1]. O => Darkest Black. 1 => Lightest white.