Is your feature request related to a problem? Please describe.
MapPalette uses a java.util.HashMap with an Integer value.
GrimAC likes keeping millions of instances loaded.

Describe the solution you'd like
As the comment above the field states,

// TODO: Can we use fastutils here?
private final HashMap<Object, Integer> stateToId = new HashMap<>();

FastUtil would work fine here, basically any implementation of an Object2IntMap.

Describe alternatives you've considered to solve your solution without
us adding this as a feature?
It's a simple change I would've submitted a pull request for if it weren't for the 5 different platforms I'd have to test and worry about shading.