title: Set transparency/saturation of palette in ggplot
body: <p>Let's start with the <code>viridis</code> palette. In my opinion, colours are a bit just too bright for me, and for my purposes they look too artificial. therefore, I would like to apply some sort of transparency or similar to reduce saturation:</p>
  
  <pre><code>library(nord)
  library(scales)
  library(viridis)
  library(nord)
  
  show_col(viridis(5))
  show_col(viridis(5, alpha=.5))
  </code></pre>
  
  <p>Applying alpha transparency internally seems to work. 
  <a href="https://i.stack.imgur.com/W7sRB.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/W7sRB.jpg" alt="enter image description here"></a>. </p>
  
  <p>However, when run in ggplot, it automatically changes alpha to 1 and plots the original viridis in full intensity:</p>
  
  <pre><code>ggplot(faithfuld, aes(waiting, eruptions)) +
    geom_raster(aes(fill = density)) +
    scale_fill_viridis(5, alpha=.5)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/DnL1r.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/DnL1r.jpg" alt="enter image description here"></a> </p>
  
  <p>In another example, I found the opposite problem, lack of intensity/saturation. For example, the "aurora" palette from the <code>nord</code> package is great, but it looks a bit faded, lacking some saturation, at least for my purposes.</p>
  
  <pre><code>show_col(nord("aurora",5))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/rr4oo.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/rr4oo.jpg" alt="enter image description here"></a></p>
  
  <p>Similarly, I tried to set alpha internally, in this case to 1, but this pruduces a different effect as compared to viridis, changing the palette.</p>
  
  <pre><code>show_col(nord("aurora", alpha=.5))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/QUbEd.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/QUbEd.jpg" alt="enter image description here"></a><br>
  Alternatively, I have set alpha as <code>alpha()</code>. However, this only changes the color names, but they look the same.</p>
  
  <pre><code>show_col(alpha(nord("aurora",5)), .5)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/FI98M.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/FI98M.jpg" alt="enter image description here"></a></p>
  
  <p><strong>How can I reduce saturation/intensity in <code>viridis</code>and increase in the <code>nord</code> palettes in <code>ggplot</code>?</strong></p>