The Cowboys have worn their whites their last two trips to Washington. And I believe 2007 was the last year the Cowboys wore their blues in Philadelphia. There are very few teams who make the Cowboys wear their blues anymore--the Panthers are the only team that comes to mind--and they don't play each other with any regularity.
"Many of the greatest things man has achieved are not the result of consciously directed thought, and still less the product of a deliberately coordinated effort of many individuals, but of a process in which the individual plays a part which he can never fully understand." - Friedrich Hayek