[Solved]-Highcharts. Series have same color after drilldown-Highcharts

You can achieve it by defining colorByPoint: true property for each series. Like this:

Highcharts.chart('container', {
    ...
    drilldown: {
        series: [
            {
                colorByPoint: true,
                name: "Chrome",
                id: "Chrome",
                data: [
                   ...
                ]
            },
            {
                name: "Firefox",
                id: "Firefox",
                colorByPoint: true,
                data: [
                   ...
                ]
            },
            {
                name: "Internet Explorer",
                id: "Internet Explorer",
                colorByPoint: true,
                data: [
                   ...
                ]
            },
            {
                name: "Safari",
                id: "Safari",
                colorByPoint: true,
                data: [
                   ...
                ]
            },
            {
                name: "Edge",
                id: "Edge",
                colorByPoint: true,
                data: [
                   ...
                ]
            },
            {
                name: "Opera",
                id: "Opera",
                colorByPoint: true,
                data: [
                   ...
                ]
            }
        ]
    }
});

Check this fiddle - https://jsfiddle.net/30o1genk/

UPD: When you add series dynamically after a drilldown, your drilldown event handler should looks like this:

 events: {
    drilldown: function(e) {
      const point = e.point;

      const series = {
        colorByPoint: true, 
        data: [{name: "test", y: 10}, {name: "test2", y: 20}],
      };


      this.addSingleSeriesAsDrilldown(point, series, false)  

      this.applyDrilldown();
    }
  }

Check this fiddle - https://jsfiddle.net/1fpdckn5/ for the dynamic case.