A shiny Lugia is not Purple A shiny Lugia is where the places on Lugia's body is normally blue or the big spot on its body is now pinkish-red. If it is pruple, It is a shadow Lugia which is only found if Pokemon XD: Gale of Darkness: Which you must purify before trading
Continuing what Gogeta said, therefore you can't get a Shadow Lugia into Ruby, Sapphire, or Emerald.