It's very simple. You can either loop through cells or directly set it like below.
this.dataGridView1.Rows[0].Cells[1].Style.ForeColor = System.Drawing.Color.Red;
If you wan't it to happen dynamically, code the below event. For example
private void dataGridView1_CellFormatting(object sender, DataGridViewCellFormattingEventArgs e)
{
if (e.ColumnIndex == 0 && e.RowIndex == 0)
e.CellStyle.ForeColor = Color.Red;
}