C#

A2 = 256 - A1;//now this would be dumb
R2 = 255 - R1;
G2 = 255 - G1;
B2 = 255 - B1;

public class MyChildForm : Form
{
  protected void ShowParentFormsProperty()
  {
    FormParent form = (FormParent)MdiParent;
    if (form != null) MessageBox.Show(form.MyProperty);
  }
}

public class MyChildForm : Form
{
  private FormParent parent;
  public MyChildForm(FormParent parent)
  {
    this.parent = parent; // use "this" when field and parameter are the same.
  }
  protected void ShowParentFormsProperty()
  {
    if (parent != null) MessageBox.Show(parent.MyProperty);
  }
}

public class FormParent : Form
{
  public FormParent()
  {
    // If you want to pass the parent in the constructor...
    MyChildForm form = new MyChildForm(this);
    AddOwnedForm(form);
  }
}