title: scale_color_manual() for different geoms in ggplot
body: <pre><code>library(tidyverse)
  
  delta &lt;- tibble(
    type = c("alpha", "beta", "gamma"), 
    a = rnorm(3, 5), 
    b = rnorm(3, 6)
    ) %&gt;% 
    mutate(delta = abs(a - b)) %&gt;% 
    gather(`a`, `b`, `delta`, key = "letter", value = "value")
  
  
  ggplot(delta %&gt;% filter(letter != "delta"), aes(type, value, fill = letter)) +
    geom_col(position = "dodge") + 
    geom_col(data = delta %&gt;% filter(letter == "delta"), width = 0.5) + 
    scale_color_manual("grey", "black", "blue")
  </code></pre>
  
  <p>I'd like the <code>a</code> and <code>b</code> bars to be grey and black. And the <code>delta</code> bar to be blue. How do I do this with <code>scale_color_manual()</code>? Seems my syntax above is off.</p>
  
  <p><a href="https://i.stack.imgur.com/puFcx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/puFcx.png" alt="enter image description here"></a></p>