If you spend some time really looking at your own skin, you'll come to realize that there are so many colors embedded within.
For those that are familiar with makeup, when buying foundation or concealer it's all about knowing if your undertones are warm, neutral, or cool (aka knowing if your skin color is more yellow, brown, or red).
It's very similar when painting! I have to get the undertones and cultivate a spectrum of values within the shades of colors used to account for highlights and shadows cast too.
It may be a lot to take in, but it all comes together to make something beautiful.