Okay, I admit it, this is pretty much taken straight from Wikipedia, but you're here because you didn't read it there first so I'll summarize.
Skin tones on emojis are not just randomly chosen by developers, but are part of a standard based on something called the Fitzpatrick scale. The Fitzpatrick scale grades skin colors 1 - 6 based on the skin's reaction to UV light. The lighter tones are at the lower end of the scale. It is from this scale, that emoji skin tones are standardized.
However there is a slight wrinkle (pun intended) in this skin scheme. Emoji standards use just a 5 point scale because 1 & 2 from the Fitzpatrick scale are combined. Plus, for electronic use, emojis also include colors for the yellow/orangey generic color that we are used to, and also a light shade of blue (that I've never personally seen before in the wild).
These days many emoji sets includes versions of all emojis crossed with each available color. Emoji generators, like the emoji maker at Editable GIFs lets you choose a skin tone while creating emojis dynamically.