I was playing pokemon Soul Silver and traveled into Cerulean Cave so i could catch Mewtwo. I finally found it but i failed many times trying to catch it. After like the 7th time when i talked to Mewtwo the battle happend normally but instead of having purple eyes and a tail his tail and eyes were green.

I tought it was a bug/glitch on the game so i turned my DS off. When i came back to try to catch it he was normal again. What was that?