CTA has been updated for the first time in a little while, now allowing use in 1.21.5 and a new feature: Colored water in cauldrons! (With thanks to eccentric_nz for the inspiration)
The new features can be used but this is not necessary, when loading the plugin it will resort to custom textures being disabled.
In the config file you can turn on custom models and with a quick reload and with the texture pack you can now have custom colored water for each potion type!