This property is used to access or change the background color
of a field.
This property is only valid if this annotation is associated
with a form field.
If you assign a color to this property the color supplied must
be grayscale, RGB or CMYK. A value of null indicates that the
background is transparent.
When you set this property a clone of the XColor you assign is
created to avoid one XColor being shared by multiple
After changing this property you will need to call UpdateAppearance to realize