Mod the Sims user Tedw was able to modify a Skin Tone in The Sims 4 Create a Sim Demo! This proves the fact that changing Custom Skin Tones will be possible in The Sims 4 with a little help of Mods.
I’m not entirely sure if this is the right thread for this – I know it’s not really news about TS4. For testing purposes, I made a default replacement of the blue skin tone in the game, changing the blue for a purple color. It shows that default replacement mods can be made for things like skin. Hopefully replacements could be made to fix issues with some of the darker skin tones lacking warmth and some of the eye colors being too similar (there are two greens and two blues that are almost identical to each other).