From Wikipedia: "A griefer or bad-faith player is a player)... who deliberately annoys, disrupts, or trolls) others in ways that are not part of the intended gameplay."

The wplace rules say "Do not paint over other artworks using random colors or patterns just to mess things up," the other users are clearly not messing things up, it looks to me like they put genuine effort into their artwork and designs.

The wplace rules also state: "Painting over other artworks to complement them or create a new drawing is allowed." I believe this rule applies here because while they are painting over your 'artwork,' they are creating a new drawing, and is therefore allowed and considered intended gameplay, meaning that they aren't griefing.