Now you see them, now you don't. Rainbow highlights can be as subtle as you want them. Here, oil slick strands are hidden beneath her layers.

Is green not your best color out of the oil slick hair bunch? That's okay. Stick with hints of blue violet and red violet shades to achieve a minimal, yet statement-making oil slick style. The opposing colors beautifully complement each other against your already-dark strands without being too in your face.