Well, Excel is actually capable of like 16 million colors (something like that).
But only 56 used at a time.
They are stored in a list of Index #s (1 - 56)
You can customize and change each of those index #s to different colors, but again, still only having 56 at a time.
To customize a color, click Tools - Options - Color, Select a color you DON'T want, then click Modify.
Notice that you sacrifice 1 color, to create a custom one yourself.
so if you take Red (index #3 by default), and change it to purple, index #3 is now purple, and you no longer have red.
That is called your Pallette. The 56 colors defined for that book.
Each workbook has it's own pallette. Each one CAN be different from the other.
so opening 1 book, it may have one set of colors assigned to the 56 indexes, while a 2nd book has something else.
How to fix it? You can restore the 56 indexes to the Default colors by..
Tools - Options - Color
Reset.