With 1.13 alot of materials that used to have subdata like terracotta now got their own Material instance which is amazing for setting blocks and creation in general but when checking for certain blocks I couldn't find a method besides hardcoding the different colored terracotta blocks into a collection.

The Tag interface was introduced for exactly that as it contains a preset collection of materials.
For 1.13 this interface could be expanded to not only contain the wool types under
WOOL or the fish items under ITEMS_FISHES but also stained glass, terracotta and the other materials that used to mostly defined through their subdata. Basically every block group that is not covered by a custom blockdata interface could be stored here.

I am not really familiar with the new API at all, and if this suggestion does not fit the new desgin philosophie im sorry in advance