This was posted by Debra Dalgleish:

This should set the default colour for any new books:

1. Create a new, blank workbook.
2. Choose Tools>Options
3. On the Colour tab, click on the yellow square
4. Click Modify, select a colour as the default, click OK
5. To make yellow available, select the square that has the same colour
as the new default, and change it to yellow
6. Click OK.
7. Choose File>Save As
8. Save as Type: Template
9. FileName: Book
10. Location -- save the file in your XLSTART folder
11. Click Save