Use the Fill property to return a FillFormat object.
The following example changes fill color for comment on cell B2.
Var Comment: TXLSComment;
begin
...
With Workbook.Sheets[1] dobegin
Comment := Range['B2', 'B2'].Comment;
if Assigned(Comment) then Comment.Shape.Fill.ForeColor.RGB := $E0E0FF;
end;
...
end;