TGtk3WSColorDialog = class(TWSColorDialog)
  protected
    class procedure SetCallbacks(const AGtkWidget: PGtkWidget; const AWidgetInfo: TGtk3Dialog); virtual;
  published
    class function CreateHandle(const ACommonDialog: TCommonDialog): THandle; override;
    class procedure ShowModal(const ACommonDialog: TCommonDialog); override;
  end;
....

class function TGtk3WSColorDialog.CreateHandle(const ACommonDialog: TCommonDialog): THandle;
var
  Widget: PGtkWidget;
begin
  Widget := gtk_color_chooser_dialog_new(PChar(ACommonDialog.Title), nil);
  Result := THandle({%H-}PtrUInt(Widget));
end;

class procedure TGtk3WSColorDialog.ShowModal(const ACommonDialog: TCommonDialog);
var
  Dlg: PGtkColorChooser;
  ColorValue: TGdkRGBA;
begin
  Dlg := PGtkColorChooser(ACommonDialog.Handle);
  ColorValue := TColortoTGdkRGBA((ACommonDialog as TColorDialog).Color);
  gtk_color_chooser_set_use_alpha(Dlg, False);
  gtk_color_chooser_set_rgba(Dlg, @ColorValue);
  gtk_widget_show(PGtkWidget(Dlg));
  {
  gtk_color_chooser_get_rgba(Dlg, @ColorValue);
  (ACommonDialog as TColorDialog).Color := TGdkRGBAToTColor(ColorValue);
  }
end;

function RegisterColorDialog: Boolean; alias : 'WSRegisterColorDialog';
begin
  RegisterWSComponent(TColorDialog, TGtk3WSColorDialog);
  Result := True;
end;