C# Winform How to change all the listbox items?

Question

I am making Color Picker Program. And I want to make my program to change all the items in listbox to RGB Format when I click RGB in Combobox And this is my code

private void mainForm_KeyDown(object sender, KeyEventArgs e)
    {
        if (e.KeyCode == Keys.C)
        {
            Point mousePoint = Control.MousePosition;
            int x = mousePoint.X;
            int y = mousePoint.Y;
            Color color = GetMousePointColor(mousePoint);
            SetColor(x, y, color);
            if (colorFormatCombo.SelectedIndex == 0)
            {
                color_list.Items.Add(hexcode.Text);
            }
            else if(colorFormatCombo.SelectedIndex == 1)
            {
                //I dont know how to change Format Hex to RGB
            }
            this.color_list.DrawItem += color_list_DrawItem;
        }
    }