There are Triton Umbras as well. I think a dev said they should be a purple breath for them prior to their death, but I am not seeing that. The only way to know for sure is to Info the Arthrons and Tritons before killing them (though I got surprised by killing a charging one with Overwatch). Two methods of dealing with them is fire (incendiary grenade or fllamethrower prior to death), or paralyzing them instead of killing them.