So it seems that no matter the method of creating custom palettes, they are treated as data-blocks within the current blend file, so what is the best way to design your palettes with the intention of reusing them in other blend files; is this another situation where we must rely on appending, and if so, what exactly are we appending? Materials?
File or F4->Append or Link->Navigate to blend file and Double click it->Palette->Select the palette you want Append/link. If it's a palette you want to always be apart of your setup you can append or add or create the palette then make sure the shield icon is blue(Set fake user). Then click File->Defaults->Save startup file. Note: Each Application template(General, 2D animation, Sculpting, VFX, Video Editing, and any you add) has it's own startup file.
Another option is to create an image with the colors you want for your color palette and then open the image in image editor and click Image->Extract palette. This will create a palette with the name of the image.
Yes, I think you're right - Palettes are datablocks that need to be appended between files for best use - If you do a File>>Append they exist in a .blend file like this:
And inside, you will see any palettes saved - either through use, or having been marked as F (for Fake User).
I have a couple of palettes I use over and over, and I have saved them in my custom preset, so that I don't have ot keep appending.