The colour scheme for the Tau needed to fulfill several criteria. I wanted it to have a nice contrast to the Kroot and the Tyranids. I also didn't want it to be too colourful. So they ended up with a scheme which suggests that they come from an ice world. Again this provided a nice difference to the bases and underlines the fact that the Tau were kidnapped by the Kroot. In every Fire Warrior squad the Shas'ui will be accompanied by a Kroot minder in case he's getting funny ideas...