This is part of the manifesto of the delightful Dispatchwork project, conceived by artist Jan Vormann in Belgium in 2010:
I don’t enjoy living in dull and grey cities. Do you? Have you noticed that toys for kids are generally very shiny and colorful? I wonder why that is, given that they are to be brought up to live in mostly dull and gray cities as adults. Since I lived in many of such cities, I am seeking to improve the appearance of public spaces in different ways, in terms of what I consider improvement. Dispatchwork aims to seal fissures in broken walls worldwide, completing the material compilation in urban constructing and adding color to the urban greyscales, by inserting a very basic construction-material: Plastic Construction Bricks (PCBs).
It has since spread worldwide: here’s an example in Piccadilly Circus in London:
See more examples on the project website.