Do they leave a stain or residue?

Nope! Gel Blasters blast biodegradable Gellets™. Gellets™ are small, colorful, water-based beads that are eco friendly, biodegradable, nontoxic and all natural. When Gellets burst on contact they instantly start to evaporate, meaning in 15 minutes or so POOF all evidence of battle is gone!

Source: GEL BLASTER SITE