It will always be impossible if you can't:
1. Make their body have a specific shape (One of the number one factors that makes them all look alike regardless of skin color)
2. Make them shorter or taller (It's even worse if they're all the same height)
3. Give them a relatively long hairstyle. The longest one goes down to the bottom of the neck.
4. Make them wear shin/knee high boots with jeans.
Just a quick note. In Saints Row 2 (2008) all of the above was possible so R* didn't really put much effort into the character creator. The only detailed part was really the mother/father heritage settings.