Hi Ladida,
Thanks for choosing Syncfusion Products.
You can achieve your requirement by setting Background property as like below,
[Sample code]
[C#]
datagrid.Model[3, i].Background = Brushes.Red;
datagrid.Model[5, i].Background = Brushes.Orange;
datagrid.Model[7, i].Background = Brushes.PaleGreen;
We could not reproduce the issue with read only property. Please find the sample that we used to test. Can you please share the sample that reproducing this issue with us, this will be much helpful for us to server you.
Sample Location:
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=WpfApplication1-860584033.zipPlease let us know if you have any questions.
Thanks,
Karthick