Yes, it is possible to sum a column of numbers based on the background color of the cells in Excel 2007. Here are the steps to do so:

1. Select the cell where you want to display the sum of the green cells.
2. Click on the "Formulas" tab in the ribbon.
3. Click on the "More Functions" dropdown menu and select "Statistical".
4. Select "SUMIF" from the list of functions.
5. In the "SUMIF" dialog box, enter the range of cells you want to evaluate in the "Range" field.
6. In the "Criteria" field, enter the formula
   Formula:

   =CELL("color",A1)=204 

   , where "A1" is the first cell in the range you want to evaluate. This formula checks if the cell background color is green (which has a color code of 204).
7. In the "Sum_range" field, enter the range of cells you want to sum if the criteria is met.
8. Click "OK" to close the dialog box and display the sum of the green cells in the selected cell.

Note that this formula only works for cells with a solid fill color. If the cells have a pattern or gradient fill, this formula will not work. Also, if you want to sum cells with a different color, you will need to change the color code in the formula accordingly.