NullEntity of derived entities of an abstract entity

public abstract class Color : IbEm.Entity
{
    /// <summary>Gets or sets the ID. </summary>
    [Key]
    [DataMember]
    [Bindable(true, BindingDirection.TwoWay)]
    [Editable(true)]
    [Display(Name="ID", AutoGenerateField=true)]
    [IbVal.RequiredValueVerifier( ErrorMessageResourceName="PEQ_LNContainer_ID")]
    public System.Guid ID {
      get { return PropertyMetadata.ID.GetValue(this); }
      set { PropertyMetadata.ID.SetValue(this, value); }
    }
}

public class Red : Color {}
public class Blue : Color {}

public class Car 
{
    // Data Properties
    public System.Nullable<System.Guid> ColorID;

    // Navigation Properties
    public Color Color { get {...}; set {...}; }

}