Until now I only spoke of VGA screens. Macintosh screens present a slightly different story. The standard Macintosh palette looks a lot like the IBM compatibles. But it is different as you can see above. But it gets worse.
Some screens can only display one palette at a time.
If you have two graphics with two different palettes, you will see only one with the right colors. To prevent problems, all graphics should use the same palette. A palette that can be used by Macintosh and IBM compatible systems. You can use a so called web palette to do this. This contains only the colors that are present in both system palettes.