led chaser fill color effect
You need two counter for this, one to keep track of how far the LED bar is filled, and one about the current 'running' LED to eventually 'touch' the LEDs which are already moved to the right.

Every led (currentLed) will run from index 0 until the last filled index, while the last filled index is decreased with 1 after every loop.

(note: I haven't checked the code).

// Fill from the right, thus count backwards.
for (int filledLedIndex = NUM_LEDS; filledLed &gt;= 0; filledLed--)
{
   // Now let a LED run from 0 upto filedLedIndex;
   for (int currentLed = 0; currentLed &lt; filledLedIndex)
   {
      // Make previous LED black
      if (currentLed &gt; 0)
      {
         leds[i - 1] = CRGB::Black;
      }

      // Make current LED red
      leds[i] = CRGB::Red;

      // Show.
      FastLED.show();
      delay(50);
   }
}

// Clear all LEDs.
for (int led = 0; led &lt; NUM_LEDS; led++)
{
    leds[i] = CRGB::Black;
}

FastLED.show();
delay(50);