lates the sRGB luma of a colour
 * @private
 *
 * @param {*} $c - Color value
 *
 * https://lnikki.la/articles/sass-better-colour-based-on-brightness/
 * http://robots.thoughtbot.com/closer-look-color-lightness
 *
 */
/**
 * Return a greyscale color based on $context--greyscale-base
 *
 * @param {Integer} $val - lightness value between $context--greyscale-min and $context--greyscale-max
 * @param {Float} $opacity - opacity value between 0.1 and 1, to one decimal place
 *
 */
/**
 * Return a color from a col