.. well yes, I think Bruno has a point there. Street art is best left in the street. Judging by the pictures from Paris there, it would seem that a clean surface is much more 'safe' than the confusing contours of graffiti, where you aren't able to distinguish what is supposed to be there, and what isn't.
I just had the most convincing revelation of the horrible but quite possible event of sneezing, where a booger, or two, is launched towards the wall attaching itself. Now, we all know the character of the booger; small, discrete, and sticky enough to last through a cataclysm. And what's even worse is that it has to be removed manually. I'm sure that we all have our individual strategies on how to deal with such events, but my point is that a single color painted surface would infact be the best choice after all.