rgbgfx -c currently supports .ACT (Adobe color table), .ACO (Adobe color swatch), and .PSP (PaintShop Pro) palettes as input. .ACT and ACO are binary formats, PSP is human-readable text (it starts with "JASC-PAL").

Other formats that might be worth supporting:

• .GPL (GIMP) [1] [2] (the default format of an editor frequently used for its palette view+edit support)
• .JSON (superfamiconv) [1] (might be convenient for superfamiconv compatibility)
• .HEX (lospec.com) [1] (a simple hex RGB text format)
• .TXT (paint.net) [1] (a simple hex ARGB text format)
• I was going to say "the same format that can be provided directly on the command line", but @palette.dat where palette.dat contains -c #aaa,#bbb,#ccc,#etc is good enough (although it doesn't allow line breaks for all that data). Supporting one of .HEX or .TXT would be nice since they use the same hex-style values, but one per line.

(See also, Tilemap Studio's implementations of those three and others)