How can I adjust the spectrum of color for my skin tones in my game?

You have to change skin tones manually for each sim, there is no way to edit multiple sims at once.

Also, just an FYI, the title and your request is dangerously close to sounding incredibly offensive to a lot of people, I would recommend a rephrasing.

:# :/ no way to limit which skin tones appear, you can use testing cheats to go to create a sim on anyone you see out and about that you’d like to change....

I thought this was super offensive at first lol, you can't adjust the skin tone range but you can edit sims however I find the diversity refreshing.

Normally I agree that I find the diversity refreshing, but the darker skin tones do not look real to me. I have a lot of black friends and they look nothing like this at all. I didn't want to come across offensive, my first language is not English and I had no other way to express what I wanted. I guess what I am trying to say is that I am looking for a way to adjust the spectrum of color in my skin tones that is more realistic for what is in real life.

In terms of changing it with CC (I'm just assuming that's what you wanted to do since this is posted in the mods and cc section), you could probably create an override that replaces the default skin tones that you don't like with ones that you do like. But I'm not sure if anyone has done anything like that and it's not really something I feel like doing myself.

Lana's CC Finds has a genetics section with several default skin options: http://sssvitlans.tumblr.com/genetics