This utilizes the UV map as of 21w10a, and will likely not be reliable for future versions of the game. This is mainly a proof of concept.
UV map was found here: https://twitter.com/Xilefian/status/1369695626921967622?s=20
Minecraft Commands Discord: https://discord.gg/QAFXFtZ
Community shaders doc for 21w10a: https://docs.google.com/document/d/18AhcnAI55liax72yh70njUomIzezOKshCurfdZPTKwM/edit#heading=h.170mpxepi1h9
This wasn't meant to be anything pretty, so you may see a bit (a lot) of repeated code in the vertex shaders. I tried to add anything that was a cutout and green to the rainbow. I also experimented with adding some waviness/wind to some objects.
Have fun!