It's great that you are trying to improve the way your characters look.
I am not great with 3d but i know a little.
What models and software are you using?
I mostly use Genesis 8 on Daz3d.
The thing I like about Genesis 8 is using Skin Builder - it likely won't come out for Genesis 9.
Skin Builder can add moles and stretch marks, change skin color, add fingernail and toenail colors and more.
It takes a little time to bake in the details but you only have to do it once for each character.
This is a sketch - the bra straps look like shit... It's a study with a young adult so I didn't go crazy aging the skin.
But you should be able to notice moles and stuff.
Looks like I used the Skin Builder blush too. I could have colored the fingernails as well or maybe I used something else.
I forget how I got the skintone but it's warmer than the default.
A lot of custom models come with their own skin textures - this is true for G9 as well as G8.
They sometimes have makeup for the face, fingernail colors and textures for the crotch.
Poses and expressions are also important in giving variety and personality.
Of course if you are using something else then this is no help (-_-) Good luck