They liked the idea of the purple hair because Thanos is "someone so simple, so intuitive" that he would of course mimic Thanos's purple skin tone in his hair color. And as for his colorful nails?

all-purple Thanos (pictured, top). On a mission to collect all six magical and McGuffiny “Infinity Stones”, Thanos plans to use these arcane artefacts to inflict his twisted will on reality.