{   
   // Commenting out the next line makes the problem disappear
   gROOT->SetStyle("Plain");
   const UInt_t Number = 3;
   Double_t Red[Number]    = { 1.00, 0.00, 0.00};
   Double_t Green[Number]  = { 0.00, 1.00, 0.00};
   Double_t Blue[Number]   = { 1.00, 1.00, 1.00};
   Double_t Length[Number] = { 0.00, 0.50, 1.00 };

   UInt_t NColors = 77; // unwanted fill color appears from 77 onwards !!!
   TColor::CreateGradientColorTable(Number,Length,Red,Green,Blue,NColors);
   THStack *hs = new THStack("hs","Stacked 1D histograms");

   TH1F *h1st = new TH1F("h1st","test hstack",100,-4,4);
   h1st->FillRandom("gaus",20000);
   h1st->SetLineColor(kRed);
   hs->Add(h1st);
   TH1F *h2st = new TH1F("h2st","test hstack",100,-4,4);
   h2st->FillRandom("gaus",15000);
   h2st->SetLineColor(kGreen);
   hs->Add(h2st);
   
   TCanvas *cst = new TCanvas("cst","cst",10,10,700,700);
   hs->Draw();

}