I am not sure if the problem is directly related to upgrading to Excel 2003,
or a setting in Excel which has been changed due to the upgrade.

The app is full of macro's as well as having VBA code. All cells are
protected except for those used for user-input. The cell in question is a
calculated value. If it exceeds a certain value it must change from red to
blue. This is done in VBA.

This has always worked, until the upgrade to 2003.

Is there a security setting which can be used to allow VBA to change the
font color of the protected cell, or am I going to have to use VBA to
unprotect the cell, change the font color, and protect the cell again?

Nigel