Paint the world

This graffiti is round the corner from where i live. It catches my eye everytime i drive or walk past. I particularly like the 'pow' ... it amuses me that the artist was so impressed by the impact of their own (that's if it was all done by the same person) work that they had to highlight it.

Paint the world ... i wonder if he had a particular colour scheme in mind?

If you could paint the world one colour, what colour would you choose?