code: function (link, element, attribute) {
          if (link.type === graph.link.LinkTypes.VALUE) {
              return "#525863";
          } else {
              var colorId = link.source.label + link.label + link.target.label;
  
              var color = provider.colorScale(colorId);
              if (attribute === "stroke") {
                  return provider.colorLuminance(color, -0.2);
              }
              return color;
          }
      }
docs: Return the color to use on links and relation donut segments.
  
  
  Return null or undefined
  @param link
  @param element
  @param attribute
  @return {*}