Why does my black car show swirls and scratches so easily?

Black and other dark colors are not necessarily softer, but they reflect light in a way that makes every fine scratch visible, because each scratch catches light against a dark background. Many dark paints are also on the softer side, so they pick up fine scratches more easily during washing and drying. Light colors hide the same scratches. The fix is the same for any color: careful contact washing with clean mitts, plenty of lubrication, gentle drying and protection that adds a slick layer. Existing swirls can be removed by polishing.