I received an excel file that has a light grey shading. But when I
copy and paste the format to an excel file that I created, the shading
comes out light yellow. I ran some code to tell me what the
Interior.ColorIndex is and it returned 19 for both the grey cell and
the light yellow cell.

What's causing the difference? Is is simply a difference in excel
versions? I am working from Excel 2003 (the person I received the
file from is unavailable so I don't know what version they are using)