Your question should be in the Mac FAQ, if there was one.
The short answer is:
1) Yes, the color table gets messed up, even if you just open and save and even if you do NO EDITING AT ALL (incredulous scream).

MacSoft should be ashamed.
2) To edit any of the PICT files related to game mechanics, download a PC scenario from the net which has the GIF file for the file you wish to edit. Edit that. Then Convert it with graphic converter. Edit only the gif file, never the converted file. So if you create a scenario, you need to keep a double set of graphics file.
Dear moderator. If this topic is moved to the Mac forum, there'll be no complaints from me.