Use a THStack to show a 2-D hist with cells with different colors. 
root > .x multicolor.C
root > .x multicolor.C(1)
  
 
  
 
void multicolor(
Int_t isStack=0) {
 
   TH2F *
h1 = 
new TH2F(
"h1",
"h1",nbins,-4,4,nbins,-4,4);
 
   TH2F *h2 = 
new TH2F(
"h2",
"h2",nbins,-4,4,nbins,-4,4);
 
   TH2F *h3 = 
new TH2F(
"h3",
"h3",nbins,-4,4,nbins,-4,4);
 
   for (i=0;i<20000;i++) 
h1->
Fill(
r.Gaus(),
r.Gaus());
 
   for (i=0;i<200;i++) {
      if (val <= 0) continue;
      } else {
      }
   }
}
 
 
 
Option_t Option_t TPoint TPoint const char GetTextMagnitude GetFillStyle GetLineColor GetLineWidth GetMarkerStyle GetTextAlign GetTextColor GetTextSize void char Point_t Rectangle_t WindowAttributes_t Float_t r
 
virtual void SetFillColor(Color_t fcolor)
Set the fill area color.
 
virtual Int_t GetBin(Int_t binx, Int_t biny=0, Int_t binz=0) const
Return Global bin number corresponding to binx,y,z.
 
virtual Int_t Fill(Double_t x)
Increment bin with abscissa X by 1.
 
virtual void SetBinContent(Int_t bin, Double_t content)
Set bin content see convention for numbering bins in TH1::GetBin In case the bin number is greater th...
 
virtual Double_t GetBinContent(Int_t bin) const
Return content of bin number bin.
 
2-D histogram with a float per channel (see TH1 documentation)}
 
void SetBinContent(Int_t bin, Double_t content) override
Set bin content.
 
The Histogram stack class.
 
virtual void Add(TH1 *h, Option_t *option="")
add a new histogram to the list Only 1-d and 2-d histograms currently supported.
 
void Draw(Option_t *chopt="") override
Draw this multihist with its current attributes.
 
This is the base class for the ROOT Random number generators.
 
- Author
 - Rene Brun 
 
Definition in file multicolor.C.