Meaning of Ability Tree Node Colors

I understand that green is spells, white is super basic things like stat changes, and red is archetype-defining major abilities, but what is the difference between yellow, pink, and blue?

 

Apparently they are supposed to correspond to item rarities, so they should go in ascending power/importance based on the "rarity" of the node (yellow is unique, pink is rare, etc.).

 

Yellow nodes are meant to be minor abilities that can enhance gameplay. They may or may not be a notable part of an archetype's moveset.
Purple/pink nodes are meant to be notable abilities that are a part of an archetype's moveset.
Blue nodes are nodes that are meant to be an integral part of an archetype.
Red nodes are meant to define the archetype as a whole.