How to change the background color of several buttons at once in windows forms?

The thing is like this: I have two forms, Form1 and Form2 .

Form1 is the main one and has 20 buttons which each represents a different table of my restaurant. Clicking on any of them opens a Form2 in which you can register orders, etc ... Each form2 also contains a Title button Abrir Mesa which I need that when clicked I change the color of button background MESA 1 in Form1 . If the button Abrir mesa of MESA 2 is clicked, then I need it to be THIS and not the MESA 1 that changes color, and so on.

I have created this that works for me only for the first button, that is to say the MESA1 but I do not manage to achieve something that is for everyone. I leave some code to see if someone can help me. Thank you very much!

   // FORM 1

  public void btnMesa1_Click(object sender, EventArgs e)
    {
        Form2 f2 = new Form2(this);
        f2.Text = "Mesa 1";
        f2.Show();
    }

    private void btnMesa2_MouseClick(object sender, MouseEventArgs e)
    {
        Form2 f2 = new Form2(this);
        f2.Text = "Mesa 2";
        f2.Show();
    }

    public void cambiarFondo()
    {
        btnMesa1.BackColor = Color.Green;
    }


       // FORM 2



    public partial class Form2 : Form
{
    double precio = 0;
    double cantidad = 0;
    double subtotal = 0;
    double total = 0;
    Form1 f1 = new Form1();

    public Form2()
    {

    }

    public Form2(Form1 parametro)
    {
        InitializeComponent();
        f1 = parametro;
    }

    private void Form2_Load(object sender, EventArgs e)
    {
        DataGridViewButtonColumn colBotones = new DataGridViewButtonColumn();
        colBotones.Name = "colBotones";
        colBotones.HeaderText = "BORRAR";
        this.Pagos.Columns.Add(colBotones);
    }

    private void txtCantidad_TextChanged(object sender, EventArgs e)
    {
        precio = Convert.ToDouble(txtPrecio.Text);
        cantidad = Convert.ToDouble(txtCantidad.Text);
        subtotal = precio * cantidad;
        etiquetaSubTotalNum.Text = subtotal.ToString();
    }

    // AGREGA UN PRODUCTO
    private void btnAgregarItem_Click(object sender, EventArgs e)
    {
        Pagos.Rows.Add(txtID.Text, txtNombre.Text, txtPrecio.Text, txtCantidad.Text, etiquetaSubTotalNum.Text);

    }

    // CALCULA EL VALOR TOTAL DE LA MESA E IMPRIME EL TICKET
    private void btnTicket_Click(object sender, EventArgs e)
    {
        for (int i = 0; i < Pagos.Rows.Count; i++)
        {
            total += double.Parse(Pagos.Rows[i].Cells["celdaSubtotal"].Value.ToString());
        }

        etiquetaTotalNum.Text = total.ToString();
        total = 0;
    }

    // CIERRA LA MESA
    private void btnCerrarMesa_Click(object sender, EventArgs e)
    {
        this.Close();
    }

    // DIBUJA "BORRAR" SOBRE LOS BOTONES DE ELIMINAR
    private void Pagos_CellPainting(object sender, DataGridViewCellPaintingEventArgs e)
    {
        if (e.ColumnIndex >= 0 && this.Pagos.Columns[e.ColumnIndex].Name == "colBotones" && e.RowIndex >= 0)
        {
            e.Paint(e.CellBounds, DataGridViewPaintParts.All);

            DataGridViewButtonCell celBoton = this.Pagos.Rows[e.RowIndex].Cells["colBotones"] as DataGridViewButtonCell;

            e.Graphics.DrawString("BORRAR", new Font("Verdana", 8), new SolidBrush(Color.Red),
            e.CellBounds.Left + 3, e.CellBounds.Top + 3);

            e.Handled = true;
        }
    }

    // ELIMINA LA FILA DE ESE PRODUCTO AL HACER CLICK EN EL BOTON "BORRAR"
    private void Pagos_CellClick(object sender, DataGridViewCellEventArgs e)
    {
        if (this.Pagos.Columns[e.ColumnIndex].Name == "colBotones")
        {
            Pagos.Rows.RemoveAt(Pagos.CurrentRow.Index);
        }
    }

    // CAMBIA EL BOTON DE LA MESA A COLOR VERDE/ABIERTA
    private void btnAbrirMesa_Click(object sender, EventArgs e)
    {
        this.btnAbrirMesa.BackColor = Color.Green;
        f1.cambiarFondo();
    }