Have been farming for specific villagers that offer specific terracotta. Once I get that villager type, I mark it with a tag (naming it that specific terracotta type), and double check the villager to make sure it is offering that terracotta, and buy 12 from it to "lock" its trade. However, upon later revisiting that villager, I find that the terracotta offering has changed to something else. 

So far has happened to a villager that was offering Green (changed to Yellow), and another that was offering Purple Glazed terracotta (also changed to Yellow). Seems to be defaulting to Yellow.

UPDATE: Found more villagers that have changed their terracotta from when I had locked them in from trading. Now it seems random colors are being substituted. 

Purple Glazed --> Orange Glazed

Cyan --> Light Blue Glazed

Brown Glazed --> Blue