private async void button1_Click(object sender, EventArgs e)
    {
        while (Visible)
        {
            for (int c = 0; c < 255 && Visible; c++)
            {
                this.BackColor = Color.FromArgb(c, 255 - c, c);
                await Task.Delay(5);
            }
            for (int c = 254; c>=0 && Visible; c--)
            {
                this.BackColor = Color.FromArgb(c, 255-c, c);
                await Task.Delay(5);
            }
        }
    }