The "color" is the type of image that was taken, either by plane or satelite. Aerials are taken in 1 of 3 formats usually....B/W, Color, or ColorIR(infared..good for vegetation, land use etc).
Not all of the images are going to be the same time periods. Large urban metropolis' normally have more flights(sometimes yearly) than smaller areas..........thats why StLoser and Dungver have the color I'm sure. It costs more.
I know a large project is underway to refly some of the midwest now. there is also a project called "the national map" that will be coming together over the next few years that will give you access to an unbelievable amount of map data for alot of areas.
Porker.........you and the other Iowanians can get newer arials and more maps than tera server on another website, that only has IA data.
http://ortho.gis.iastate.edu/
use the "map search" tool. the color IR is 2002