title: Matplotlib scatter plot with two colors for only two series in dataframe
body: <p>I have a dataframe with two series and no categories. I want to plot the scatter-plot for x and y with different colors.</p>
  <p>I tried <code>plt.scatter(obs_flow, sim_flow,c=('red','blue'), alpha=0.3)</code> but gave an error :</p>
  <p><code>ValueError: 'c' argument has 2 elements, which is inconsistent with 'x' and 'y' with size 3954.</code></p>
  <p>How do I define the colors in this case when no category is available? Here's how my dataframe looks like:<br />
  <img src="https://i.stack.imgur.com/eZWtF.png" alt="" />.</p>
  <p>EDIT :
  The plot without any color argument passed is looking like this:<br />
  <img src="https://i.stack.imgur.com/Bo5lY.png" alt="" /></p>
  <p>Can we have different colors for observed and simulated points?</p>