You don't want the Dark Side to change how you look? Enslave it, make it your weapon just like a lightsaber or sword. The Jedi are the ones who want to live in harmony and then there are sith like [member="Darth Sinna"] who see the force as something greater then they are and that it should fill them (correct me if I am wrong, your views are complex) Darth Ares looks at the force as nothing more than a means to an end. The force he enslaves, but he he follows what the dark side wants which is war. Thus his apprentice is not changed unless he wants it to. He normally looks like the picture exactly and his eyes are brown, but he can change his eyes to sith eyes at will (well it is usually when battle is about to start and he becomes excited) I do this because of how he is made. A lot of sith want to look scary to inteminate someone, but Ares wants to look like he is an easy target, like he is weak so when someone sees him they think he is easy pickings and then he kills them. Much like a flytrap that looks like a flower to trick flies and then SNAP dead.
As far as I am aware the star wars canon is not crystal clear about the effects of the dark side so I am basing it off SWTOR where there is a button to turn it on and off and thus that is what Ares does.