public partial class Form1 : Form
    {
        Button btn;
        TextBox txt;
        public Form1()
        {
            InitializeComponent();

            //a textbox
            txt = new TextBox();
            Controls.Add(txt);

            //a button to change

e.Graphics.FillRectangle(new SolidBrush(BackColor)

        public Form1()
        {
            InitializeComponent();
            this.DoubleBuffered = true;
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            e.Graphics.FillRectangle(new SolidBrush(BackColor), 0, 0, 50, 50);
            base.OnPaint(e);
        }