Detects a dark color based on luminance W3 standarts ( https://www.w3.org/TR/WCAG20/#relativeluminancedef ). It has these options: * is a hex color dark * is an Imagick pixel dark * is an Imagick pixel from a blob dark * is an area in a blob over a dark background (uses Imagick for it too). An example practical aspect: it can be useful to understand will a black colored text be visible or not over an area.