In most places, graffiti is illegal. No matter what reasons for laws against it, or if you agree or not: it simply is illegal.
But cleaning isn't. So there are some artists that are creative cleaners. For instance, it might be known that cars tend to leave loads of dirt on tunnel walls. And through partially removing the dirt, a piece of reverse graffiti (or grime writing) can come into existence. The example above is done by the Braziliaan Alexandre Orion in a tunnel in Sao Paulo. So there's really no paint involved, just clean water and a sponge.