C# How would I fill a Form1 Parameter in the same seperate class

public partial class Form1 : Form
{
    SimonFunctions sf = new SimonFunctions(this); //Added

    public Form1()
    {
        InitializeComponent();
    }

    private void StartButton_Click(object sender, EventArgs e)
    {
        sf.Colorplaff(); //call Colorplaff from Form1;
    }
}

class SimonFunctions 
{
    public static List<int> ColorNums = new List<int>();
    Form1 mainForm; //added

    public SimonFunctions (Form1 form) //Constructor added
    {
        mainForm = form;
    }

    public void ColorPlay(int x) //remove static and Form1 argument
    {
        mainForm.ButtonNum = x; //use mainForm
        if (x == 1)
            mainForm.GreenButton.BackColor = Color.Lime;
        else if (x == 2)
            mainForm.RedButton.BackColor = Color.Red;
        else if (x == 3)
            mainForm.YellowButton.BackColor = Color.Yellow;
        else if (x == 4)
            mainForm.BlueButton.BackColor = Color.Blue;
        mainForm.timer1.Start();
    }

    public void Colorplaff()
    {
        ColorPlay(1);
    }
}