Wow, they intentionally made it gray. You don't want a bright lively world because obviously it's not

D.C. Wasteland was so dull and grey because it got Nuke-raped, this was China's enemy capital and wanted to ensure that if the US survived that they'd be severely demoralized by the fact their glorious capital was in ruins.
There is no such thing as a Green Nuclear Apocalyptic event atomic bombs don't produce rainbows and make it rain skittles. In the Fallout Universe the great war did not make the world Skyrim!

-.-_-.- that is fallout New Vegas and doesn't apply as he was bitching bout the grey DC wasteland the west coast had in almost every vault a GECK that aided in plantlifr revival and the fertility of soil

Dude if you did enough exploring in fallout 3 you would of noticed they mentioned that only major vaults got gecks.The issue though was the enemy's found out before hand the Americans were building vaults and adding Gecks in certain vaults. This caused them to drop nukes quicker then they planed to. So the last bunch of vaults which were around dc had to rush. This was the only reason one geck was in dc because they planed for a bunch more since it was an area they planed to use them rebuilding.Point is there was not only one geck,it was only given to pretty much 1%..