Foreach(GridViewRow row in GridView1.Rows)
{
TextBox txt=(TextBox)row.FindControl("textBoxID");
if (txt.Text=="")
{
txt.ToolTip="you have to enter data inside text box";
txt.BorderColor=System.Drawing.Color.Red;
}
else {
txt.ToolTip="thank you for your decisions";
txt.BorderColor=System.Drawing.Color.Gray;
}
}