VBA code for Traffic light system

I have a worksheet with a series of boxes and data entry points that are completed by the user in a specific order. We operate our environment with manual (F9) calculation updates and would prefer this to continue, however we need this latest sheet to recalculate after some of the steps prior to moving onto the next step (so that new data can be brought up based on previous data entered). Although most of the calculations are fairly quick, the more complicated ones can take a few seconds and there is a risk that the user enters data prematurely in the next box stopping the calculation process. I would therefore like to have a traffic light system on my worksheet to show:

a) when the worksheet is calculating (Red Light)
b) when I need to press F9 to update the calculations (amber light) and
c) a Green light to say that the worksheet is calculated / up-to-date.

I have excel 2010 and windows 7. I am new to vba and have desperately tried to find a code for this but cannot find anything in existence. Could anyone help me with a vba code (or a cell formula) to display this?

Your help and advice would be really appreciated