To become Yellow Sonic, use the Debug Mode & Super Sonic cheats. Only make Sonic play. Then, start the game. Turn into Super Sonic. Then, spawn a TV thing using Debug Mode. Brake the TV thing, you'll be transported to the beginning of the level, but you won't Super Sonic, you'll be Yellow Sonic!(witch means you can get hurt)

P.S. Also, if you try to turn into Ashura, you won't. Instead your eyes might become blue!