My Sa'cea Tau were always supposed to be night fighters. Sa'cea is the closest thing to a "hive world" that the Tau Empire has, and in one paragraph in the third ed codex it mentioned that they were skilled night fighters. But the grey armour Sa'cea wear is very close to Tau flesh, and I wanted helmet less Tau to stand out, so I figured out what Tau, Kroot, and Vespid would look like with severe vitamin D deficiency (and this is back when Tau had cyan, cobalt-based blood) so my Fire Warriors are an unhealthy grey-green, the Kroot are a really dark green-brown, and my Vespid are a calcinated beige/white. It's explained that they spend too much time in the training grounds, and too little under mandated-use UV lamps. Also, bright orange and white markings aren't the best for night fighting, so the Cadre uses a deep red and grey instead. But the Ethereal attached didn't know this before deploying with them, so is covered in garish orange and has a healthy skin tone, which actually helps them blend visually with the army when it's on the board, but still stand out.