How to save?

The easiest thing to do would be to define a user setting and use that to store and retreive the color.  To create a user setting, in the solution explorer, under properties, double click on settings.settings.  When the settings appear, name the setting shown to something descriptive like 'UserBackColor'.  Since the type to store is a color, set the type to System.Drawing.Color.  Next, set the scope to User (so the user can control the setting), and set a default value for the setting using the color picker shown when you click in the value box.

Now, since the setting is created and you want to allow the user to set the color and you want open the form back up using the color set by the user, you will need to retrieve the color in the form's constructor and set the form backcolor to that value after the initialize component call. 

  public Form1()

        {

            InitializeComponent();

            this.BackColor = Properties.Settings.Default.UserBackColor;

        }

And next, you will want to allow the user to set the color (and update and save the property as the same time):

        private void backColorToolStripMenuItem_Click(object sender,   

        EventArgs e)

        {

            if (colorDialog1.ShowDialog() == DialogResult.OK)

            {

                this.BackColor = colorDialog1.Color;

                Properties.Settings.Default.UserBackColor =

                colorDialog1.Color;

                Properties.Settings.Default.Save();

            }

        }