title: add_trace: control the color
body: <p>I have a plot where I have first trace in gray that will be overplotted by other traces in colors. My issue is that in <code>plotly</code>-version 4.7.1. as well as in version 4.8.0. I can't adjust the color. </p>
  
  <p>One year ago this code would work: </p>
  
  <pre><code>mysim=data.frame(x=rep(1:4,4),y=rbinom(16,10,0.5),id=rep(1:4,each=4))
  
  my_colors&lt;-c(             ## add the standard plotly colors
          '#1f77b4',  #// muted blue
           '#ff7f0e',  #// safety orange
           '#2ca02c',  #// cooked asparagus green
           '#d62728'  #// brick red
               ) 
  
  
  plot_ly() %&gt;%
   add_trace(x=1:4,y=rbinom(4,10,0.4),type='scatter',mode='lines',
              line=list(color='#CCCCCC',dash='dashed'),hoverinfo='skip',opacity=0.25) %&gt;% 
    add_trace(data=mysim,x=~x,y=~y,type='scatter',mode='lines', split=~as.factor(id),
              line=list(color=my_colors),hoverinfo='skip',opacity=1) 
  </code></pre>
  
  <p>Sadly I do not have that machine anymore. But it seems that there were changes made to <code>plotly</code> since then. I also tried using the <code>color</code> argument instead of <code>split</code> and used <code>colors</code> instead of the <code>line</code>-list to specify the colors. It didn't have any impact. I still get this plot: <a href="https://i.stack.imgur.com/zNIHp.png" rel="noreferrer"><img src="https://i.stack.imgur.com/zNIHp.png" alt="enter image description here"></a></p>
  
  <p>What am I missing here? How can I make it work? </p>