getContrastRatio
calculate the contrast value between this color and the given color contrast value is accordingo to // see http://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
calculate the contrast value between this color and the given color contrast value is accordingo to // see http://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef