In the first book of the LoTR, after his fight with the Balrog of Moria, Gandalf the Grey changed to Gandalf the White.
After dying fighting the Balrog of Moria, Gandalf was "sent back" as Gandalf the White by Eru.
Here is an analysis of Gandalf's death and resurrection, but let me extract the relevant sentences from letter #156 (emphasis mine):
"For in his condition it was for him a sacrifice to perish on the Bridge in defence of his companions
He was handing over to the Authority that ordained the Rules, and giving up personal hope of success.
Gandalf really 'died', and was changed [...] He was sent by a mere prudent plan of the angelic Valar or govenors; but Authority had taken up this plan and enlarged it, at the moment of its failure. 'Naked I was sent back- for a brief time, until my task is done'. Sent back by whom, and whence? Not by the 'gods' whose business is only with this embodied world and its time; for he passed 'out of thought and time'. Naked is alas! unclear. It was meant just literally, 'unclothed like a child' (not disincarnate), and so ready to receive the white robes of the highest.
Hopefully this shows that it wasn't the Valar who "sent Gandalf back", but a higher Authority "who ordained the Rules", and also that Gandalf was literally naked before this Authority, who gave him the "white robes of the highest".
This Authority above the Valar can be none other than Eru.