Setting GoComment background color

The GoComment object defaults to a light yellow background. How can I change this?

I've overriden CreateBackground() but not sure on what properties to change to modify the fill color.

The documentation for GoComment.CreateBackground includes the actual definition provided by GoComment. You need to set the Brush.
I hope you also realize that you don’t have to override CreateBackground–you can just set ((GoShape)comment.Background).Brush to any simple brush. The advantage of doing it your way is that you don’t have to worry about copying that code everywhere you might want to create that kind of GoComment.