Hi James,
You need to set the grid.IgnoreReadOnly to true, before you try to programatically change any style properties for cells that are readonly. After making the changes to the cell, set the ReadOnly property of the particular cell to false/true and again set back the IgnoreReadOnly property to false.
Please refer to the following KB link for more information on setting properties for the ReadOnly cells.
http://www.syncfusion.com/support/kb/grid/Default.aspx?ToDo=view&questId=338Regards,
Rajagopal