title: How to assign colors to circles in matplotlib?
body: <p>Somehow, assigning colors to circles works different from assigning colors in scatter plots:</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  fig = plt.figure(figsize=(6,6)) # give plots a rectangular frame
  
  N = 4
  r = 0.1
  
  pos = 2.*np.random.rand(N,2) -1
  
  # give different points different color
  col = 1./N*np.arange(0,N) 
  
  # Method 1
  for i,j,k in zip(pos[:,0],pos[:,1],col):
      circle = plt.Circle((i,j), r, color = k)
      fig.gca().add_artist(circle)   
  plt.show()
  
  # Method 2
  plt.scatter(pos[:,0],pos[:,1], c = col)
  plt.show()
  </code></pre>
  
  <p>Why does Method 2 work while Method 1 gives the following error:</p>
  
  <pre><code>ValueError: to_rgba: Invalid rgba arg "0.0"
  to_rgb: Invalid rgb arg "0.0"
  cannot convert argument to rgb sequence
  </code></pre>