new_contents: url = 'http://blogs.mathworks.com/images/steve/2010/mms.jpg'
  
  import os
  if not os.path.exists('mm.png'):
      print "Downloading M&M's..."
      import urllib2
      u = urllib2.urlopen(url)
      f = open('mm.png', 'w')
      f.write(u.read())
      f.close()
  
  
  from skimage import io, color, exposure
  import numpy as np
  
  
  mm = io.imread('mm.png')
  mm_lab = color.rgb2lab(mm)
  L, a, b = mm_lab.T
  
  left, right = -100, 100
  bins = np.arange(left, right)
  H, x_edges, y_edges = np.histogram2d(a.flatten(), b.flatten(), bins,
                                       normed=True)
  
  
  import matplotlib.pyplot as plt
  from matplotlib.widgets import RectangleSelector
  from matplotlib.patches import Rectangle
  
  f = plt.figure()
  ax0 = plt.subplot2grid((2, 2), (0, 0))
  ax1 = plt.subplot2grid((2, 2), (0, 1), rowspan=2)
  ax2 = plt.subplot2grid((2, 2), (1, 0))
  
  f.suptitle('Select values by dragging the mouse on the histogram')
  
  ax0.imshow(mm)
  ax0.set_title('Input')
  ax0.set_xticks([])
  ax0.set_yticks([])
  
  ax1.imshow(exposure.rescale_intensity(H, in_range=(0, 5e-4)),
             extent=[left, right, right, left], cmap=plt.cm.gray)
  ax1.set_title('Histogram')
  ax1.set_xlabel('b')
  ax1.set_ylabel('a')
  
  rectprops=dict(
      facecolor='gray',
      edgecolor='white',
      alpha=0.3
      )
  selected_rectangle = Rectangle((0, 0), 0, 0, transform=ax1.transData,
                                 **rectprops)
  
  ax1.add_patch(selected_rectangle)
  
  result = ax2.imshow(mm)
  ax2.set_title('L + masked a, b')
  
  
  def histogram_select(e_click, e_release):
      x0, y0 = e_click.xdata, e_click.ydata
      x1, y1 = e_release.xdata, e_release.ydata
  
      x0, x1 = min(x0, x1), max(x0, x1)
      y0, y1 = min(y0, y1), max(y0, y1)
  
      selected_rectangle.set_xy((x0, y0))
      selected_rectangle.set_height(y1 - y0)
      selected_rectangle.set_width(x1 - x0)
  
      green_mm_lab = mm_lab.copy()
      L, a, b = green_mm_lab.T
  
      mask = ((a > y0) & (a < y1)) & ((b > x0) & (b < x1))
      green_mm_lab[..., 1:][~mask.T] = 0
  
      green_mm = color.lab2rgb(green_mm_lab)
  
      result.set_data(green_mm)
      f.canvas.draw()
  
  rs = RectangleSelector(ax1, histogram_select, drawtype='box',
                         spancoords='data', rectprops=rectprops)
  
  plt.show()
  
subject: Add Lab color selector demo.