Add one more default line color
Making the default set of line colors not be a multiple of the default set of symbols would make it easier to distinguish more lines on a chart. At the moment red lines always have triangle symbols. By adding one more color each time the first color was reused it would be on a different symbol.
Alan Hinton shared this idea
You can set any number of colors: http://jsfiddle.net/highcharts/rj8efe0j/