I have some npc sims that spawned in my game with a custom skintone that I didn't want to make use of and I also wanted to use the alien eyes for some other sims that exist in my hood, but I'm not sure how to do this.

This may help, and also this post.

If you're comfortable with using it, SimPE is a way that you can do this.

Load your neighbourhood (Tools --> Neighbourhood Browser) and go into the Sim Description. Select the sim you want and go into the More tab; you should see an option called SimDNA. Select that. You'll see a bunch of jumbled letters and the like; the code. The information is easy to find online, though.

You can find information on the code of each of the Maxis tones here:
https://sims.fandom.com/wiki/Genetics

I don't know if there are any mods that make this easier, someone more mod savvy would probably have to clear that up. I hope you found this helpful in some capacity; it's the only way I know to go about altering skins anyway.

Be careful which NPCs you're altering though, as some can cause corruption when edited.

Christianlov's Skintone Selector makes it possible for almost any sim to wear almost any skintone. You only have to make the sim selectable (this won't work for some of the non-social NPCs). It's purely cosmetic and won't affect the sim's genetics, but if the NPC sims in question aren't going to breed anyway then that shouldn't be a problem.

The Skintone Selector can be downloaded either from my signature or from the page for Christianlov's Sim Cloner (they seem to be the same version). The painting can switch between Maxis skintones without doing anything else to it, but if you want to add custom skintones to the painting, you'll also need the CPack Editor. Download link and instruction link for the Cpack Editor are also in my signature.

Thanks for the replies, I was able to change the skin successfully.