I think it's a clear red and won't agree with any argument for yellow but I can see how someone may see it as yellow.
The boot throw very much looks like an act intended to show dissent and displeasure in the earlier decision . Intent to hurt or injur is not there in the boot throw. It is deliberately hit/thrown 'at the ground' (in the referee's direction) and the force is just enough for the boot to reach the referee or just go past him.
Having said that, intent, the fact the player is injured and the foul that injured him were missed are very small factors here. He has committed an act in anger which is seen as an act of violance towards the referee. Contact is irrelevant and if that is not dealt with a red card, a precedence is set for the rest of the game.