There are five skin tone modifiers available for human emoji in Unicode.
They are encoded as U+1F3FB through U+1F3FF and are named Emoji Modifier Fitzpatrick Type-1-2, Type-3, Type-4, Type-5, and Type-6. When placed after a compatible human emoji, they change its displayed skin tone; alone, they appear as colored swatches.
The system is based on the Fitzpatrick scale, a dermatological classification developed by Thomas B. Fitzpatrick in 1975 to describe how skin responds to ultraviolet light. Unicode combines Fitzpatrick types I and II into the lightest emoji modifier, producing five modifiers from six scale categories.
A common mix-up is counting six because the underlying Fitzpatrick scale has six types. Unicode provides five emoji modifiers, not six, and the default yellow appearance is not an additional skin-tone modifier. These modifiers were introduced with Unicode 8.0 in 2015 as part of a broader effort to represent human diversity in emoji.