Every color for the terracotta blocks

This is working as intended as of now. You have to put in stained clay, and then use the modifiers (which the syntax thing calls old_replace or something like that) You have to do the same thing for logs or some planks. I'm sure it will eventually be changed to have every block as it's own unique item in those lists, but for now, this is just how it is.

This is the correct command:
/fill ~ ~ ~ ~ ~ ~ stained_hardened_clay ["color" = "blue"]