void twopal()
{
   TCanvas *c1 = new TCanvas("c1");
   TH2F *h1 = new TH2F("h1","h1",40,-4,4,40,-4,4);
   TH2F *h2 = new TH2F("h2","h2",40,-4,4,40,-4,4);
   Double_t a,b;
   for (Int_t i=0;i<5000;i++) {
      gRandom->Rannor(a,b);
      h1->Fill(a-1.5,b-1.5);
      h2->Fill(a+1.5,b+1.5);
   }
   TExec *ex1 = new TExec("ex1","grey();");
   TExec *ex2 = new TExec("ex2","gStyle->SetPalette(1);");
   h1->Draw("colz");
   ex1->Draw();
   h1->Draw("col same");
   ex2->Draw();
   h2->Draw("col same");
} 

void grey()
{
   Double_t Red[2]   = { 0.00, 1.00};
   Double_t Green[2] = { 0.00, 1.00};
   Double_t Blue[2]  = { 0.00, 1.00};
   Double_t Stops[2] = { 0.00, 1.00};

   Int_t nb=50;
   TColor::CreateGradientColorTable(2,Stops,Red,Green,Blue,nb);
}