Continuing the discussion from Guinea Pigs Wanted!:
If you have the skills and equipment, you could replace the three LED “dropping” resistors with ones of higher value, to lower the maximum brightness.
However, might I suggest a physical solution: Place some type of neutral desity filter inside the front of the case, above the RGB LED.
In the past, I’ve used pieces of the grey/silver Mylar anti-static bags that electronic parts come in, stuck on with clear double-sided tape.
Another thought is to use one or more layers of a white self-adhesive address label cut to size, or just plain white paper with clear double-sided tape.
You could even use a piece of black electrical tape to cover the area above the LED. You would still be able to see the LED light “bleeding” through the edges of the case and reflecting off of the circuit board (especially with a white Arduboy).