To have 255 colors in the histograms, add the following to the root logon macro:

const Int_t NRGBs = 5;
const Int_t NCont = 255;
Double_t stops[NRGBs] = { 0.00, 0.34, 0.61, 0.84, 1.00 };
Double_t red[NRGBs]   = { 0.00, 0.00, 0.87, 1.00, 0.51 };
Double_t green[NRGBs] = { 0.00, 0.81, 1.00, 0.20, 0.00 };
Double_t blue[NRGBs]  = { 0.51, 1.00, 0.12, 0.00, 0.00 };
TColor::CreateGradientColorTable(NRGBs, stops, red, green, blue, NCont);
gStyle->SetNumberContours(NCont);

and for python:

from array import array 
def set_palette(name="palette", ncontours=999):
    """Set a color palette from a given RGB list
    stops, red, green and blue should all be lists of the same length
    see set_decent_colors for an example"""

    if name == "gray" or name == "grayscale":
        stops = [0.00, 0.34, 0.61, 0.84, 1.00]
        red   = [1.00, 0.84, 0.61, 0.34, 0.00]
        green = [1.00, 0.84, 0.61, 0.34, 0.00]
        blue  = [1.00, 0.84, 0.61, 0.34, 0.00]
    # elif name == "whatever":
        # (define more palettes)
    else:
        # default palette, looks cool
        stops = [0.00, 0.34, 0.61, 0.84, 1.00]
        red   = [0.00, 0.00, 0.87, 1.00, 0.51]
        green = [0.00, 0.81, 1.00, 0.20, 0.00]
        blue  = [0.51, 1.00, 0.12, 0.00, 0.00]

    s = array('d', stops)
    r = array('d', red)
    g = array('d', green)
    b = array('d', blue)

    npoints = len(s)
    TColor.CreateGradientColorTable(npoints, s, r, g, b, ncontours)
    gStyle.SetNumberContours(ncontours)