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):
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.