Hi Steve,

For a start: I'm interested in the reason of your question. Why would
you want this?
It looks like an 'impossible question' because there are no answers
up to now. I'll give it a try. Any readers ... please comment if it is a
silly approach.

The bad part of my answer:
The color palette is stored by workbook. You found this yourself:
changing Book.xlt doesn't change the palette of existing workbooks.
So changing the palette in Personal.xls won't work either. And the
Workbook_Open event only deals with Personal.xls itself.

To good (and difficult) part:
In your Personal.xls, you could create a class module to program
the 'global' Excel Workbook_Open event. There, you can change the
yellow default into grey for each and every workbook that is opened.
And ... set grey to yellow, I think.
(I don't know if it would be possible to change Excels 'habit' to pick
grey i.s.o. yellow.)
The command line would be
 ActiveWorkbook.Colors(6) = RGB(192, 192, 192)
but you'll have to exclude Personal.xls itself from this action (because
it is a hidden workbook?).

Search the web on how to create the class module to interfere with
the Excel events. There are nice samples.

Last remark: I think PowerPoint remembers the last used fill color
by presentation. Excel doesn't.

Frans