Install this plugin, the latest dev build of Lib's Disguises, and ProtocolLib > 3.4.0.
To get the green skin, type /green.
To get the yellow skin, type /yellow.
Guess how you get the red and blue skins.
If you want to add new skins to this, see the file get_blob_and_sig.lua.
The existing skins used in the plugin can be seen here: https://gist.github.com/riking/0561d308e287b3ef9835