title: Change color of leaflet marker
body: <p>Is there anyway to change the color of leaflet marker base on the value of some variable. In the following map, for example, I wish to assign marker color based on mag variable:</p>
  
  <pre><code>library(leaflet)
  
  data(quakes)
  
  # Show first 20 rows from the `quakes` dataset
  leaflet(data = quakes[1:20,]) %&gt;% addTiles() %&gt;%
    addMarkers(~long, ~lat, popup = ~as.character(mag))
  </code></pre>