You can download additional level masks for use with PL Tool's lighting preview feature.
For the preview to work, a set of two images is required: a fullbright image and a mask image that will be used for palette/color selection. This page has some pre-made masks (taken from Dreamcast levels on PC) that you can use in PL Tool.
Download level masks (open folder)
If you create a "masks" folder in the folder with your PL files and put the masks there, PL Tool will attempt to automatically load the corresponding masks when you load a PL file.
If you are working on a custom level, you need to create your own lighting masks. Here are some mods that can help with mask creation:
Test Spawn by x-hax - this mod can bypass all menus and load any level from command line.
Lighting masks mod to facilitate mask creation.