Palette Editor
"This is the MegaZeux Palette Editor including mode 0 and 1 and 2 and 3. It is provided as a MegaZeux world file (no other files are needed), and is in the public domain. (I have used some others but I didn't find it very good, so I made the new one; I am unlikely to need it much (generally the standard PC palette is good enough) but now I have in case it can help.)"