With the Landscape Palette (bigf-#.dat), you can tell Populous which colours it should use to draw the terrain itself. From left to right it should be keep to the darker colours so it can display the side of the hills correctly. From top to bottom, it begins with the water colours, then it keeps moving to the highest point of the hill. This is only a colour palette, do not put textures and other stuff into it as it will mess up the terrain. To add a texture, use the Displacement instead. Here is a little preview how the Landscape Palette transforms into the terrain:

On this preview the palette has been flipped to show you how the terrain changes.

Since the Cliff Palette only uses the first 128 colors to make the Lava and Damaged Ground effects, only use the first 128 colors in this palette. If you use other colours, it will load Memory Garbage and the display will end up weird, and different in every game launch.