title: chose color to use with sf.colors
body: <p>I'm building a static map using the <code>sf</code> package and I want my  map to have both a specific color and transparency.  I can manage to get both independently but not together.  </p>
  
  <pre><code>library(sf)    
  nc = st_read(system.file("gpkg/nc.gpkg", package="sf"), quiet = TRUE)
  </code></pre>
  
  <p>I can make the map all red simply:</p>
  
  <pre><code>plot(nc["SID74"], col="red")
  </code></pre>
  
  <p>produces: 
  <a href="https://i.stack.imgur.com/mI5l5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mI5l5.png" alt="enter image description here"></a></p>
  
  <p>If I want to add transperency, I have to change the <code>col</code> argument and pass it a <code>sf.colors</code> call:</p>
  
  <pre><code>plot(nc["SID74"], col=sf.colors(n=1, alpha=0.3))
  </code></pre>
  
  <p>produces:</p>
  
  <p><a href="https://i.stack.imgur.com/Vc7Xm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Vc7Xm.png" alt="enter image description here"></a></p>
  
  <p>I have the transparency but it's the default blue and I  can't find the option to change it to red in <code>?sf.colors</code>.  I want to set the color and also have the transparency.</p>
  
  <p>Any idea how?</p>