The Kotor games have a nice system where the darker you choices get the paler your skin and yellower your eyes the phrase Psychotic urges is uttered a lot, which is the practical and simple way to show that attuning yourself to either side has consequences.
Yellow eyes, pale skin dried skin, white hair and a thinned figure are usual signs of dark side influence, but you must first surrender yourself to darkness, then understand it and finally accepting it.
A great conversation in the Plagueis book reads:
Sidious watched the last of the brush fires burn out, then said, “Will Ieventually be physically transformed?”
“Into some aged, pale-skinned, raspy-voiced, yellow-eyed monster, youmean. Such as the one you see before you.” Plagueis gestured to himself,then lowered himself to the ground. “Surely you are acquainted with the lore: King Ommin of Onderon, Darths Sion and Nihilus. But whether it will happen to you, I can’t say. Know this, though, Sidious, that the power of the dark side does not debilitate the practitioner as much as itdebilitates those who lack it.” He grinned with evil purpose. “The power ofthe dark side is an illness no true Sith would wish to be cured of.”
