In excel, cannot figure out why background color for a cell will not change?

Did you check for conditional formatting? It is harder to change.

Not sure where it is at in Excel 2007, but the mac version has it under:

Format => Conditional Formatting

Update: From the comments we have a way for Excel 2007:

Ah ha! I found a condition string for that cell. The condition formatting option is under the "Home" tab of the ribbon, not in data or formulas. – Troggy