From the Super Mario Wiki, the Mario encyclopedia
Not to be confused with Shiny.
A Flashy Hammer sticker

Flashy is a sticker variation in Paper Mario: Sticker Star. Flashy Stickers are even stronger than Shiny stickers. All Jump and Hammer stickers except Burn/Chill/Baahammer, the Fire and Ice Flowers, the POW Block, the Shell and Mushroom stickers have Flashy variations. They look almost exactly the same in the overworld as Shiny stickers, but can be identified in the Album by having a diagonal rainbow-colored background.

Flashy stickers are rarely found in the overworld. Some of them (specifically the Flashy variants of Jump, Clone Jump, Hammer, and Fire/Ice Flower) are sold in shops; rarely, a Buzzy Beetle or Spike Top may drop a Flashy Shell sticker. Flashy stickers are commonly found in World 5.

Flashy stickers are surpassed in strength and size by the Big Shiny, and Megaflash stickers.