Capitol Hill Uniform?

I noticed last week that a lot of men wear the same thing- khaki/tan pants with a blue buttoned down shirt. So I tested my theory out and between yesterday afternoon and lunch time today, I saw many examples. They walked, rode bikes, and came and went in all shapes and sizes - but that all had the same thing in common. Do they all know something that we don't? Does it represent anything in particular? Beats me... but it was everywhere. Now that is the power of observation...