When detonated, they release a cloud of black, gray, and red smoke, which transforms any living creature who inhales it to an Eevee. Usually, EeveeBombs turns its target into an Eevee with alternate pelt colors, and sometimes even gives them features never usually seen in Eevees. For example, Frosti is a silver Eevee with blue markings, and has two floating wings.
SkyWarrior, another member of Rt. 50, created a comic series revolving around the EeveeBombs. These comics aren't considered canon, as any user who's turned into an Eevee doesn't remain that way in Rt.50 for long. However, there's a possibility that this just means that the effects of EeveeBombs wear off over time.