Ashura was the fan-given name of the green Sonic in Sonic 2. He's not a real character, just a glitch in the colour pallet accessible via the debug code. If you place too many waterfall sprites in Emerald Hill(they look invisible when in debug), you can mess up the palate and Sonic turns green/black. Just make sure it's done at the start of the level by a waterfall, or it'll take a little longer for the sprites to overload.
~ Crazy C Lea & SA Tails