Tools, Customize, then Right click on the "icon" in the toolbar and
select "Edit button image".

Mrs. Fischer wrote:

I remember that it is possible to change the colors in some icons. For
example, I had a former boss that recommended that I change the 'delete cells
icon' to have red cells within the icon so that I could quickly click the
button without thinking 'is this the delete or insert icon?' I can notice red
is deleting and green is inserting cells rather than having only the
preprogrammed color icon. How do I customize icons by chaning their color on
Excel 2003?