title: Set color for missing values in folium choropleth
body: <p>I have a dataframe with <em>some</em> countries and variables and I would like to produce a choropleth map with <code>folium</code>, using a geojson file for the entire world. I have a problem with <code>folium</code> assigning maximum value on a color scale to countries that are not present in my dataframe. Minimum exaple below:</p>
  
  <pre><code>import random
  import pandas as pd
  import folium
  import json
  
  map_data = pd.DataFrame({
      'A3':['POL', 'CZE', 'SVK', 'HUN', 'AUT'],
      'value':random.sample(range(10), 5)
  })
  
  m = folium.Map(
      location = [50, 15], 
      zoom_start = 4
  )
  
  m.choropleth(
      geo_data = 'https://github.com/simonepri/geo-maps/releases/download/v0.6.0/countries-land-10km.geo.json',
      data = map_data,
      columns = ['A3', 'value'],
      key_on = 'feature.properties.A3',
      fill_color = 'YlOrRd'
  )
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/ZFACc.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ZFACc.png" alt="enter image description here"></a></p>
  
  <p>My question is the following: How can I tell <code>folium</code> to <strong>assign a specific color (e.g., gray or transparent) to missing countries</strong> (i.e., those present in json file but not in <code>map_data</code>), instead of coloring them as a maximum value for given variable (which is a strange behavior)?</p>