Rainbow is Richie Blackmore's band, the founding guitarist of Deep Purple. But yeah, Dio was in Rainbow for a long time, before Graham Bonnet & Joe Lynn Turner. Dio went from Rainbow to Black Sabbath for the Heaven & Hell & Mob Rules albums, before forming DIO. Dio in Rainbow
Sorta. They were originally established with Ronnie James Dio's American rock band Elf, but after their first album, Blackmore fired the backing members and continued with Dio until 1979.