In the Netherlands it is often already hard to see the actual livery of a train through all the dirt on the outside, so that obscures any graffity as well
Especially the case for NS, where even transparent windows are getting rare, the regional operators have much cleaner trains.
Any revised stock in the Netherlands is now being liveried using coloured foils, so graffity is easier removed as it involves replacing a bit of foil in the worst case (avoiding the need to use agressive cleaners which may permanently damage the paint)