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

The thing is like this: I have two forms, Form1and Form2.

Form1It is the main one and it has 20 buttons, each one representing a different table in my restaurant. Clicking on any of them opens a page Form2in which orders can be registered, etc... Each form2also contains a Title button Abrir Mesawhich I need, when clicked, to change the background color of the button MESA 1in the Form1. If the button Abrir mesais MESA 2clicked, then I need it to be THIS and not the MESA 1one to change color, and so on.

I've created this that works for me only for the first button, that is, MESA1but I can't find something that works for everyone. I leave some code to see if someone can help me. From already 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();
    }