Based on the comment at https://www.curseforge.com/minecraft/mc-mods/abyssalcraft?comment=663, I'm considering making hardcoded colors (to begin with) accessible to resource pack makers.

For anything numerical or alphanumerical there will be a JSON file in the assets folder (exact location TBD) containing the various values to be changed. The resource pack maker will simply need to supply a modified version of said file in their resource pack to override any specific value (as the file will be parsed on every resource reload).

The dread spawn, greater dread spawn etc textures will also be split up into separate files for more customization. They're also pretty much the only mobs that re-use the same texture.

There are probably more things that I haven't though about that could be handled in a similar fashion, so do let me know if something you think should be possible to change in a resource pack isn't in the list.

Things that will be more tweakable

• Crystal colors will be changeable
• Dimension sky colors will be changeable
• Biome colors will be changeable (grass, foliage and water at least, maybe sky as well, depending on the biome?)
• Potion colors maybe?
• The dread spawn, greater dread spawn etc will have individual textures

As with any issue tagged with development, any input and/or feedback is appreciated, or you could just bitch about it on Reddit (and I might come across it at some point in time).

Putting this here for those that might stumble upon this issue in the future:
https://github.com/Shinoow/AbyssalCraft/wiki/For-resource-pack-makers