From left to right: Gray-breasted, Purple, Gray-breasted, Purple, Gray-breasted, Brown-chested, Purple and Brown-chested!
Note the dark forehead of the Gray-breasted and the pale, frosty forehead of the Purple. The thin shaft streaks on the Purple can be hard to see on this image but shows better on the individual Purple Martin images. Also note the cleaner white underside of the Brown-chested with the black dots coming down the center of the breast.