Try this: Open a blank workbook and change the fill color to the green you want. Then 'Save As' and name the file 'Workbook'. Navigate to Applications/Microsoft Office 2011(or your version)/Office/Startup/Excel and click Save. Then open your finder and navigate to the folder you saved the Workbook file in. Find the file, 'right' click and select 'Get Info', and delete the '.xltx' file extension from the name of the file.
Click OK (it might say 'Remove') on the warning message. In theory, this should replace the 'blank' workbook that opens when you open Excel. I haven't personally tried this, so your mileage may vary. I also don't know if you'd need to 'Save As' every time you save a new file.
To specify an alternate startup folder, on the Excel menu, click Preferences, click. If the workbooks that you want to use are stored on a network drive, or if you don't want to. The next time that you open Excel, it opens a blank workbook.