title: Change grouping color with highcharter
body: <p>I have a barplot with different categories, I customized the bar color for each category but I dont know how to change the under graph point color</p>
  
  <p>My data :</p>
  
  <pre><code>library(highcharter)
  library(dplyr)
  
  x = structure(list(stat = c("abandonne_pro", "admissible", "admissible", 
                              "admissible", "non_recu", "refuse", "refuse", "en_cours", 
                              "non_recu"), 
                     annee_recep = structure(c(17532, 17532, 17897, 18262, 17897, 17532, 17897, 17897, 17532), class = "Date"), 
                     nb_stat = c(4L, 57L, 112L, 1L, 2L, 5L, 5L, 1L, 1L)), 
                row.names = c(NA, -9L), 
                class = "data.frame",
                .Names = c("stat", "annee_recep", "nb_stat"))
  </code></pre>
  
  <p>Add the color variable :</p>
  
  <pre><code>x = x %&gt;%
    mutate(
      color = case_when(
        stat == 'abandonne_pro' ~ '#87CEEB',
        stat == 'admissible' ~ '#7CFC00',
        stat == 'non_recu' ~ '#FF7F50',
        stat == 'refuse' ~ '#FF0000',
        stat == 'en_cours' ~ '#FFFF00',
        TRUE ~ '#C0C0C0'
      ),
      date = format(annee_recep, format="%Y")
    )
  </code></pre>
  
  <p>The Graphic :</p>
  
  <pre><code>hchart(
    x,
    "column", 
    hcaes(x = annee_recep, y = nb_stat, group = stat, color = color)
  ) %&gt;% 
    hc_plotOptions(column = list(stacking = "normal"), type = "datetime") %&gt;% 
    hc_xAxis(
      title = list(text = 'Date'),
      type = 'datetime'
    ) %&gt;% 
    hc_yAxis(
      title = list(text = "Nombre de chantier")
    ) %&gt;% 
    hc_add_theme(hc_theme_gridlight())
  </code></pre>
  
  <p>The output :</p>
  
  <p><a href="https://i.stack.imgur.com/BHoSW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/BHoSW.png" alt="enter image description here"></a></p>
  
  <p>Thanks for help !</p>