title: Color contrast setting on dataLabels style parameter using Highcharts 5 is not working on some colors
body: <p>Below is a simple chart. With datalable style settings. Notice the lighter colors are setting to white (when they should set to black) making it an item of complaint with my clients because they can't see them well.</p>
  
  <p><strong>Here is the code (note the style parameter block):</strong></p>
  
  <pre><code>$(function () {
  Highcharts.chart('containera', {
    chart:{ type:'column' },
    xAxis: {
      categories: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
    },
    plotOptions: {
      series: {
      colorByPoint: true,
        dataLabels: {
          enabled: true,
          inside:true,
          style: {
            fontWeight: 'bold',
            color: 'contrast', 
            textShadow: false      
          }
        }
      }
    },
    series: [{
        data: [29.9, 71.5, 106.4, 129.2, 144.0, 176.0, 135.6, 148.5, 216.4, 194.1, 95.6, 54.4]
    }]
    });
  });
  </code></pre>
  
  <p><strong>Here is the Fiddle (also in the fiddle it is ignoring the textshadow:false):</strong></p>
  
  <p><a href="http://jsfiddle.net/franktudor/98Ldn972/" rel="nofollow noreferrer">http://jsfiddle.net/franktudor/98Ldn972/</a></p>
  
  <p><strong>Here is the image (note the green and orange):</strong>
  <a href="https://i.stack.imgur.com/kok9g.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kok9g.png" alt="enter image description here"></a></p>
  
  <p>What are the thresholds for contrast? Can they be altered as a global? Because that would be handy. I have 200 charts would like to fix what the highcharts engine considers light and dark colors for contrast labels globally if possible.</p>