mathews suggestion

void setup()
{
  FastLED.addLeds<WS2811, DATA_PIN, RGB>(leds, NUM_LEDS);

  pinMode(pm, INPUT);

  EVERY_N_MILLISECONDS(100)
  {
    if (up)
      paletteIndex++;
    else
      paletteIndex--;

    if (paletteIndex >= 255)
      up = 0;
    else if (paletteIndex <= 0)
      up = 1;
    }
}
//------------LOOP-------------

void loop()
{

  // other stuff
  if (lastIndex != paletteIndex)
  {
    CRGB colour = ColorFromPalette(myPal1, paletteIndex, maxBrightness, LINEARBLEND);
    fill_solid(leds, NUM_LEDS, colour);
    FastLED.show();
    lastIndex = paletteIndex;
  }

  // other stuff...
}