TCanvas *canvas[4];
Bool_t cState[4] = {kFALSE,kFALSE,kFALSE,kFALSE};
void SetBackground(TObject *canv);
Bool_t GetBackground(TObject *canv);
  
void AddToggleToContext(){
  TClass *cl;
  TList*l;
  TClassMenuItem* mi;
  
  char name[20];
  for(int n=0;n<4;n++){                               //make 4 canvases names "MyCanvas_0,1,2,3"
    sprintf(name,"MyCanvas_%d",n);
    canvas[n] = new TCanvas(name,name,n*100,0,100,100);
  }

  //add the menu item as illustrated in tutorial $ROOTSYS/tutorials/gui/customContextMenu.C 
  cl = canvas[0]->IsA();
  cl->SetContextMenuTitle("Modified TCanvas Context Menu");
  l  = cl->GetMenuList();
  mi = new TClassMenuItem(TClassMenuItem::kPopupUserFunction,cl,"Set Background","SetBackground",0,"TObject *",0);
  //mi->SetToggle();  //menu item doesn't appear if I comment this in 
  l->AddFirst(mi);
  
}

void SetBackground(TObject *canv){
  int index;
  sscanf(canv->GetName(),"MyCanvas_%d",&index);      //Get the index from the object name
  cState[index]=!cState[index];                      //toggle the state
  canvas[index]->SetFillColor(cState[index]);        //set the fill color
  canvas[index]->Draw();
  canvas[index]->Update();
}

Bool_t GetBackground(TObject *canv){
  int index;
  sscanf(canv->GetName(),"MyCanvas_%d",&index);      //Get the index from the object name
  return  cState[index];                             //return the state
}