ok you need the following
1.a pikachu that is rid of all moves, if you cant delete them then just take away what you can or you can give it all hm moves then get them removed.
2.a gameshark, ps i never used it for this game except for wierd color gliches and suburb moves.
ok you go on the gameshark actuly you dont need a pikachu just put that you want to find a pikachu put in that you want it to be all the colors it will say something like error want to contenoue click yes then this is where it gets wierd.you will go on and see certain pikachus on their some will be smeared by the colers but after a while it will combine into the pikachu and became the RaInBoW PiKaChU! theres also some glich where you can get a rainbow one a diffrent way by screwin g up the daycare center to take three of diffrent colers and them all mating.....this is easier.