Thank you Mike, that worked fine when I removed the lines:
If .Value Then
.Interior.Color = RGB(0, 255, 0)
Else
.Interior.Color = RGB(255, 0, 0)
End If
I will lose the colours, as I do not know how to set them (green for Y and red for N), but that is OK, I am happy with the Y/N instead of True/False.
Bertha