Here's a random thought: The other beast tribes (generally) have a colour difference in their clothing. Sylphs don't wear clothes. What if the tempered sylphs dye themselves (maybe the same way you can dye roses/other flowers?) to show devotion to their lightning god (purple being the colour sign of lightning in XIV)? That way there would be no immediate physical change to them either.
Might be a bit of a stretch though.