They look like that because they are pre-war buildings falling apart, and they just happened to be more colorful then normal.Even the early screenshots and trailers showed a world that looked like it was getting truly rebuilt. http://www.recoilweb.com/wp-content/uploads/2015/06/Fallout-4-3.jpg Look at the buildings, they look like they're actually being repaired properly, something we see in New Vegas even. What did we get instead? Well we got absolutely no explanation as to why the buildings look like that.
And none of those look like they are being repaired, they lack any sort of scaffolding or anything to show actual work being done on them.