For Caucasians, I usually mix nutmeg brown and white, and then tint it slightly with yellow. I will then occasionally add other colors, like more white, red, brown, yellow, etc. to make it lighter or darker, as-needed, for a specific character/ethnicity. For African Americans, I usually use nutmeg brown straight-out-of-the-bottle, but also tint that lighter-or-darker as needed.
Most of the things I make are inhuman monstrosities though, so my custom mixed bottle of skin tone doesn't actually get used that much.
But, yeah, like Pock63 said, it's arguably easier to just buy a bottle, in the shade you want, rather than futzing around with mixing your own (although making your own tends to be more economical).
Skintones are mostly a mixture of brown and pink although they can also include yellow depending on what your going for. That said it's much easier to buy skin tone color paint then it is to mix it. Every brand of model paint I know of has atliest three shades of skin tone so you have a lot to choose from
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Most of the things I make are inhuman monstrosities though, so my custom mixed bottle of skin tone doesn't actually get used that much.
But, yeah, like Pock63 said, it's arguably easier to just buy a bottle, in the shade you want, rather than futzing around with mixing your own (although making your own tends to be more economical).