change the properties of the datagridveiw from the property window.
or you can try color dialog :
private void dataGridView1_CellContentClick_1(object sender, DataGridViewCellEventArgs e)
{
ColorDialog clr = new ColorDialog();
FontDialog f = new FontDialog();
if (f.ShowDialog() == DialogResult.OK && clr.ShowDialog() == DialogResult.OK)
{
dataGridView1[e.ColumnIndex, e.RowIndex].Style.Font = f.Font;
dataGridView1[e.ColumnIndex, e.RowIndex].Style.ForeColor = clr.Color;
}
}