You'll need to have done all applicable interactions with that particular enemy to be able to play test fight against it.
The blue button, "seen", is for seeing the enemy once. (Note that as of current, this progress is only saved once you go through a door.)
The pink button, "fed", is for befriending the enemy once via the endosomatic heart upgrade that you can buy at one of the shops.
The green button... Is a secret interaction. Here's a hint: It has a large role in hardcore difficulty!