You can use following:
protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
if (GridView1.SelectedIndex > -1)
GridView1.SelectedRow.CssClass = "blue";
}
OR
protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
foreach (GridViewRow row in GridView1.Rows)
{
if (row.RowIndex == GridView1.SelectedIndex)
{
row.BackColor = ColorTranslator.FromHtml("#A1DCF2");
}
else
{
row.BackColor = ColorTranslator.FromHtml("#FFFFFF");
}
}
}