The Istari were all originally Maiar in service of the Valar. Manwe sent them to middle-earth to
Quote:
amend the errors of old, especially that they had attempted to guard and seclude the Eldar by their own might and glory fully revealed...
~ Unfinished Tales part IV/II (the Istari)
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In essence, they were a means of subtle divine intervention. Their bodies were real, could grow and be damaged and killed, whereas before all the Maiar and Valar had assumed bodies as and when they chose. The Istari were in form like men but had far greater power, and grew old only very slowly.
As for the colours, there isn't any passage (or I've never found it [img]smilies/smile.gif[/img]) that states The blue wizard is stronger than the brown wizard etc. The only time this comes into play is where Saruman the White was the greatest of the council, until he had left that role to become Saruman of Many Colours, and Gandalf subsequently stepped into the role of White.