在DataGridView控件的dgv_RowPrePaint事件dgv_RowPrePaint里添加以下代码即可，可以设置该单元格的字体、背景色等样式。
private void dgv_RowPrePaint(object sender, DataGridViewRowPrePaintEventArgs e)
{
try
{
decimal num = Convert.ToDecimal(this.dgv.Rows[e.RowIndex].Cells[9].Value.ToString());
decimal yj = Convert.ToDecimal(txtYJ.Text.Trim());
if (num < yj)
{
this.dgv.Rows[e.RowIndex].Cells[9].Style.ForeColor = Color.Red;
}
}
catch
{
}
}