This way
int i = 1;
string color = "#f89020";
foreach (DataRow row in dt.Rows)
{
if (i == 1)
{
color = "#ff0000";
}
if (i == 2)
{
color = "#00ff00";
}
if (i == 3)
{
color = "#000011";
}
PieChart1.PieChartValues.Add(new AjaxControlToolkit.PieChartValue
{
Category = row["ShipCity"].ToString(),
Data = Convert.ToDecimal(row["TotalOrders"]),
PieChartValueColor = color
});
i++;
}