Thank you for writing.
Unfortunately, the desired appearance cannot be achieved. You cannot create visual representation of rows that do not exist into in RadGridView and you cannot apply such a customization to the blank area under the existing rows. The only customization of this space than can be applied is the BackColor
property of the TableElement, however, this will be applied to the whole TableElement, not just the blank area:
.radGridView1.TableElement.BackColor = Color.Red;
Should you have any other questions, I will be glad to assist you.
All the best,
the Telerik team