Sorry, I updated my post while you were replying. It's a bad habit of mine.
I realised you were not using FastLED and removed references to it.
The atan2() function will return a Hue angle between -Pi and +Pi. Use Pythagoras to calculate the Saturation value and set Value to maximum. Then convert HSV to RGB. There are libraries and code samples around the net for that: Google for "Arduino HSV to RGB".