Highstock navigator should inherit color from baseSeries
By default, the navigator series's color is always blue, as documented here: http://www.highcharts.com/stock/ref/#navigator--series
It seems natural to me that the color of the navigator series should be the color of the baseSeries that is being shown in the navigator.
Kyle Getz shared this idea
This is supported since v6, simple demo: https://jsfiddle.net/BlackLabel/qmsp7ntw/