Digimon Tamers (2001)

S1E9 Not as Seen on TV

Following the confrontation with Devidramon, Takato attempts to revert Growlmon back to Guilmon, but his efforts prove fruitless. As the rain begins to fall, he feels hopeless about restoring his beloved partner. However, when the downpour finally ceases, Growlmon transforms back into Guilmon, captivated by the sight of a rainbow.