Biome-dependent vegetation that doesn't have its own icons and uses a rendered 3D model instead is always gray. It looks like your mod uses some sorts of heightmaps only.

For comparison, in vanilla Minecraft the same vegetation is displayed correctly (e.g. in inventory), with some specific color. Maybe you could see how Mojang did it and implement it in your mod as well.

Screenshot showing this issue:

This issue occurs on the following configuration (I didn't test other configs):

• Minecraft version: 1.16.4
• Bongo version: 1.0.4
• LibX version: 1.0.6
• Forge version: 35.1.4

This is a problem with LibX so the issue moved there. See #2