This will be solved in the next beta, so when you customize the palette it will belong to the level, not the terrainbank entry. This way you can go back to the default list, or use your custom one.
PC SPECS: Windows 8.1 Pro 64-bit, Intel Core i7-5930K (PASSMARK:13645), NVIDIA Geforce GTX 980 GPU (PASSMARK:9762) , 32GB RAM