public Form1() {
            InitializeComponent();
            TransparencyKey = TransparencyColor;
        }
        

        private void button1_Click(object sender, EventArgs e) {
            panel1.BackColor = TransparencyColor;
        }

        private static readonly Color TransparencyColor = Color.Red;

------解决方案--------------------
在随机的地方绘制一块区域，填充TransparencyKey的颜色