The inside story behind those KATY PERRY- JOHN MAYER fireworks!

GIRLY-man John Mayer set off fireworks with his sweetie Katy Perry – when she caught him borrowing a bottle of her nail polish!

“John is almost obsessed with looking good, and he doesn’t have the least problem using Katy’s grooming products to get his handsome on,” an insider told The ENQUIRER.

But the 35-year-old rocker’s habit of dipping into Katy’s stash blew up when the “Fire­work” singer, 28, learned Mayer had helped himself to a bottle of her favorite nail lacquer, sources say.

“Until then, Katy had been a good sport about John’s metrosexual ways,” the in­sider said.

“BUT things came to a head when she went to use her favorite nail polish and found the cap was loose and the polish had dried out!

“Katy doesn’t care about having to replace the polish, but she did have it out with John for being so careless.”

Pals say Katy is “completely and totally in love” with the reformed playboy, and the “Waiting On the World to Change” singer/song­writer has reportedly been shopping for an engagement ring for his lady love of six months.

And with Mayer’s reputation as a metrosexual – who reportedly dyes his hair and uses facial powder to cover up skin problems – he’s a per­fect match for the open-minded “I Kissed A Girl” singer, sources say.

“Their tiff over the nail polish ended happily enough,” added the source. “John promised to stay away from Katy’s cosmetics – but not before jotting down the color of her nail polish for his assistant!”