That's why you shouldn't base calculations on formats! Changing a format
doesn't trigger a calculation. I'm not real knowledgable about VBA but you
can insert a line of code at the beginning of your UDF that makes it
volatile:

Application.Volatile

This STILL will not cause it to recalculate when the format is changed but
it will recalculate ANY time a sheet calculation takes place.

I don't know why the experts don't discourage calculations based on
color/format.

Biff