Hello,
The fish seems healthy.
The yellow phases in Arothron meleagris (golden puffer) is a phasis, which means he can be gold and then become black with white spot again.
In the nature, I have often notice that the golden puffer is the dominant in a group (probably a male) and that the dominated are always black with white points.
Therefore, you should not start any treatment. Is the fish dominated at the moment? The specimen you have seems young.
Rudy