Cell color changing based on Time passed

Hi,

I am developing a new Unit at work and I need to be able to highlight how long a call has been with us.

Basically, a call is made by a member of the public and we then arrange for someone to visit. Due to other commitments we may not always be able to keep the original appointment.

I therefore want a cell within an excel spreadsheet to change colour as time goes on. For example for a cell to be green for 2 hours, then amber for 2 hours and then red for 2 hours. The principle being that the visit needs to become a priority if the cell goes red, or we need to call the person back.

Any ideas how I achieve this?

Thanks

Christine