I created the attached palette file that has the standard 256 Garmin colors. It can be used with programs like IconXP to create custom xpm graphics.
The same colors as in the link below. The colors are a lot easier to select when the palette is only 8 squares wide because it does a better job of showing the progression. When displayed 16 wide like on the web page you need to zig zag your eye back and forth to see the next darker or lighter color.
http://www.deltadelta.de/garmin_colours/ (http://www.deltadelta.de/garmin_colours/)
I think the 256 color pallete only applies to old models like the 60csx, eTrex Legend, etc. The newer models support 16 bit color. I used to worry about compatibility with these old models when making maps with 16 bit colors, but I always test them on my 60csx and don't see all that much difference. The 60csx just picks the closest 8 bit color.