Marker or Sharpie
First try Guardsman Fabric First Aid Stain Remover Level 1 cleaner ($3, guardsman.com); apply sparingly to a cloth (not the stain) and work in. If the stain persists, apply Level 2 solvent. Follow with a damp towel; blot excess moisture. For walls, use dish soap and a wet soft, non-cellulose sponge, which shouldn’t harm paint.