RGB LED inverted logic Arduino
In a common cathode LED, the common lead goes to the ground. The three individual colours are taken high to turn them on. That's positive logic.

In a common anode LED, the common lead is the positive, so is taken to +5V. The individual colours must now be taken low to turn them on. That's negative logic. It's not a function of the Arduino, it's a function of the LED and its connection.