Is there a way to have more than 12 RGB colours in a pallet? I tried to edit a pallet file manually and then import it but, while the pallet will import I am unable to change any colours if there is more than 12.

This make not sense.
More than 12 colors can you create for a AT5 Palette.

A RGB palette works differently. If you select a first and a last color, an your lake is eg 30 m deep, the programm will use 30 gradations (and not 12) from this palette colors.

If you want tu use more than 2 colors, follow the pics: