When working with automated annotations, the background brush (the brush that fills the interior) of most objects can be changed by changing the object's "Fill" property.
The following class objects support the Fill property:
Programmatically, you can determine if an object supports a fill, stroke or text by querying the AnnObject.SupportsFill, AnnObject.SupportsStroke, and AnnObject.SupportsFont properties of the AnnObject class. The AnnObject class contains common drawing information shared by all annotation objects. Support for annotation brushes is provided by the AnnBrush, and AnnSolidColorBrush classes. Please note that the AnnHiliteObject object does not use a fill. Change its color programmatically by changing the HiliteColor property.