Villians: Nick and his brother.
Toward the end, Shaggy and Scooby try to trick the warlock by disguising themselves as tennis players. All Shaggy did was wear a pair of powder blue shorts with his regular green shirt. Shortly afterward, when Scooby stumbles and starts riding on the warlock's demon dog, Shaggy is seen pursuing them wearing his maroon pants. After the villains are caught, Shaggy is again in the blue tennis shorts.
Nick: We would've succeeded if it weren't for you meddling kids.