private void button_Click(object sender, EventArgs e)
    {
        ColorDialog color = new ColorDialog();
        if (color.ShowDialog() == DialogResult.OK)
        {
            Button button = (Button)sender;

            int r, g, b;
            r = color.Color.R;
            g = color.Color.G;
            b = color.Color.B;

            // RGB 色值
            string rgbString = r.ToString("X2") + g.ToString("X2") + b.ToString("X2");

            //颜色字符串 Alpha + RGB
            string colorString = color.Color.A.ToString("X2") + rgbString;

            //按钮显示文本
            button.Text = "#" + rgbString;

            //按钮背景色
            button.BackColor = color.Color;

            //按钮字体颜色（ARGB），为按钮背景色的反色
            string foreColorString
            = "FF" + (255 - r).ToString("X2")
            + (255 - g).ToString("X2")
            + (255 - b).ToString("X2");

            //按钮字体颜色
            button.ForeColor =
            System.Drawing.Color.FromArgb(int.Parse(
            foreColorString, System.Globalization.NumberStyles.HexNumber));
        }
    }