The same thing happened to my G1 Vroom. After the H2O2 it came out a minty white. I put it in a place where it was in complete darkness, away from light, humidity, heat and humanity but it still re-yellowed!

It seems that it is just the 'nature' of the plastic to yellow. You can only 'reset it' with the H2O2 but that won't prevent the yellow from coming back.