FastLED/NeoPixel examples that are non-blocking?

Using Arduino LEDs and Multiplexing
25

If you want to fade back to the original colours then in place of this:-

if(step > totalSteps) step = 0; // start again

You could use this, again not tested:-

if(step > totalSteps) {
  redIncrement = -redIncrement;
  greenIncrement = -greenIncrement;
  blueIncrement = -blueIncrement;

step = 1; // don't calculate the increments again but start at first change
}

By simply making the increments the negitave of what they were, you should be able to fade back to the the original colour.