More likely than not, most people, even if they lean to the dark side, are not full dark side.
Think about all the bad things people have done in history. Check out what slavery has done through out history. You're also basically killing indiscriminately. Being evil for evils sake. Dark Side 5 is that.
Also, they have the ability to hide it, look at SI storyline. Though that would also leave one wondering why the pureblood sith wouldn't revel in being turned white to prove just how darkside slanted they are.
Also, if the movies are any indications, the darkside taint we see is basically a game feature. Palpatine didn't look the way he did because he was dark side, he looked the way he did because he had his own force lightning turned back on him.
The game just made a way for evil people to look different