The colour of the kerbals' skin: one colour for everyone, a different one for each kerbal, or your own choice for any one of them. Only the skin changes; hair, eyes, mouths and suits stay as they are.

For Kerbal Space Program 1.12.x. Needs Keystone, a separate download (on SpaceDock): this mod does not load without it.
Copy the KerbalSkins folder from the download into your KSP's GameData, beside Keystone.
Open the Keystone window (the keystone button on the game's toolbar, or Alt-K; Option-K on a Mac), turn to Kerbal Skins and switch on Change skin colours. Set one colour for everyone, let each kerbal have their own, or click a kerbal's colour box to choose theirs. A kerbal's own colour is remembered by name, in every save.
Windows 11 and Mac: tried. Linux: never run (it is plain C#, with nothing of one system's in it).
More: the long description (everything in full, and what was tried on each system) and the rest of the wiki.
CC BY 4.0: use it, change it and pass it on as you like, as long as you credit Ishiaki and link back here.
Raw stats are from the beginning of time until now. Each follower and download entry represents one hour of data. Uneventful hours are omitted.