Ahem. That's the pre-war part. Before the Earth was scorched.
Comments like this make me believe that people are forgetting that parts of it were pre-war flashbacks, and the more vibrantly colorful parts of the vid were of the flashbacks; particularly those autumn leaves. Humanity in the FOverse did try to annihilate itself in October, remember, and that's autumn in the northern hemisphere.
When the BGS devs did research for FO3, they quickly realized that true atomic bombs would pretty much reduce everything to rubble at best. Washington DC would be scoured clean of any building being even remotely intact... which would not make for a very interesting game. I chalk the color up to this sort of fudging to make a more visually appealing color palette.
This whole discussion is yet another Catch 22 decision from the looks of it, along with quest markers, fast travel, and quest items from even before then (for the TES series as well). They tinted FO3/NV with green/orange and people took issue with the washed out bleak coloring. They add color back in and remove the tint screen being on all the time, and... well.