My guess, since they all seem to have team color set as a light blue or teal, is that he took his time and made each wisp an individual unit and gave them different flying heights, while giving them team color. Or he used them as doodads and tinted the wisp model to a color he wanted. If the wisp was a doodad untinted, it'd come out looking grayish.
Very nice terrain work. Pretty accurately made as well, so plus rep!
~Craka_J