New method Int_t TPaletteAxis::GetValueColor(z) to
return the color index of the given z value.
This funtion should be used after an histogram has been plotted with the
option COL or COLZ like in the following example:
h2->Draw("COLZ");
gPad->Update();
TPaletteAxis *palette =
(TPaletteAxis*)h2->GetListOfFunctions()->FindObject("palette");
Int_t ci = palette->GetValueColor(30.);
Then it is possible to retrieve the RGB components in the following way:
TColor *c = gROOT->GetColor(ci);
float x,y,z;
c->GetRGB(x,y,z);
This function is used by TPaletteAxis::GetBinColor().