title: How to color bars in grouped plotly bar chart
body: <p>So, I want to create a bar chart with Plotly.express in Python 3.9.
  I created the following table with pandas:</p>
  <pre><code>      idx  cat1  cat2  cat3   cat4
  0       A  0.98  0.93  0.550  1.00
  1       B  0.92  0.80  0.865  1.00
  2       C  0.91  0.87  0.955  0.96
  </code></pre>
  <p>and I have the following code:</p>
  <pre><code> # df is the pandas table
  fig = px.bar(df,
          x = 'idx',
          y=['cat1', 'cat2', 'cat3', 'cat4']
      )
  
  colors = ['#dc1a22', ] * 12
  colors[2] = '#000000'
  fig.update_traces(marker_color = colors, marker_line_color='rgb(8,48,107)',
                    marker_line_width=1.5, opacity=0.6)
      
  fig.update_layout(barmode='group')
  </code></pre>
  <p>I want to color all bars in all groups in #dc1a22 except the one at third position (colors[2]).</p>
  <p>The reality is, that all bars in the first and second group are in #dc1a22 and all bars in the third group are in #000000.</p>
  <p>Is there a possibility to color a single bar in a grouped chart?</p>
  <p>Might be important: In the future I am trying to color the bars depending on their values.</p>
  <p>(0-0.5 -&gt; red, 0.5-0.9 -&gt; yellow, 0.9-1 -&gt; green)</p>