Why does the color palette change in Excel?
 

Sometimes, when I select a fill color for certain cells in Excel, after I've
saved and closed the file, when I reopen the file, the fill colors of some of
those cells has changed. When I click the drop-down arrow next to the
paintcan "Fill Color" button on the toolbar, I discover that the palette has
changed completely. It has gone from the usual rainbow-gradient layout to a
completely random layout, and the colors on the palette are not the same as
before either. As a result, the cell colors have changed, but the position
of the selection square hasn't changed. (e.g., if the cell was "light pink"
before - lower-left corner - it is "bright yellow" now, because "bright
yellow" is now occupying the lower-left corner position of the palette) How
do I make this stop?