Originally posted by SpaceWeasel
In answer to your question, I think any graphics editing program will work. To be absolutely sure, I would maintain the exact same palettes (the way to do this varies from editing program to editing program), but at the very least make sure the "special" colors stay the same - especially the magenta color used for transparency. Remember that the game decides which colors to use for special things by their palette slot, NOT their color, so just using the same shade of magenta won't necessarily work. Likewise, if a color you use for something you want not to be transparent ends up in magenta's slot, those areas will show up transparent.
On a related note, has anyone tried editing the game's interface artwork at all? I created a suave new diplomacy screen for a very ambitious mod I'm working on, which is currently limited by the game's restricted editing capabilities and lack of a scenario editor. However, I have yet to try it in the game. Has anyone noticed any weird problems with using edited artwork? It *seems* like it should be straightforward, but lots of things with Civ3 editing seem straightforward that aren't. Also, has anyone tried creating new FLCs for the leader heads? Even still ones? And any tips on maintaining pallete integrity in Photoshop? I haven't worried about pallete matching since like version 4 so I might not know how to do it anymore =P.