I'm pretty sure it's just 'further in the infrared' but I don't have xenology with me. However, the idea that the Tau buildings are covered with things we can't notice is intriguing. Their art form, "Fio'Sorral', is intricate artwork that is found all over Tau buildings. The description of this art is found in Kill Team where the humans are said to 'commonly ignore' or don't even see it as it is a form that exists so perfectly with the surroundings. The Tau describing it says that it is a perfect and physical represent of what the 'Tau'va' means- change/adaptation/and acceptance of destiny.
Here's a compiled list that I've made whenever GW has brought up Tau and colors:
Violet: The color of mourning
Silver: Sacrifice for the Tau'va
So, maybe the absence of color could be considered their warning sign if, as Tastyfish brought up, auras and colors exist more profoundly. Maybe instead of 'warning lights' the lights on a ship or base would dim slightly and have the same 'OMG' affect that we have when responding to danger. Meanwhile urgency, 'yellow' for us, might be an increasing of light, almost a pulse, in order to draw more attention.
Just thoughts- fun to think about- thanks Sussexlad for making my noggin do some joggin
And at least you're one of the hobbyists trying to learn, you'll be better off for it IMHO.