How to change color of the stacked bar charts in amchart

You can change the bar's color by setting a particular graph's fillColors property, i.e.

AmCharts.makeChart("chartdiv", {
  // ...
  "graphs": [{
      "fillColors": "#008800",
      "fillAlpha": 1, //must also be non-zero value; valid values are decimal values <= 1
       // ...
  },
  // repeat for each graph
  ],
  // ...
});

Here's a modified version of the graph that changes the Europe segment to green.

Amchart 4 chart color scheme can be set like so:

chart.colors.list = [
  am4core.color("#845EC2"),
  am4core.color("#D65DB1"),
  am4core.color("#FF6F91"),
  am4core.color("#FF9671"),
  am4core.color("#FFC75F"),
  am4core.color("#F9F871")
];

Amchart 4 piechart color scheme can be updated like this:

pieSeries.colors.list = [
  am4core.color("#845EC2"),
  am4core.color("#D65DB1"),
  am4core.color("#FF6F91"),
  am4core.color("#FF9671"),
  am4core.color("#FFC75F"),
  am4core.color("#F9F871"),
];

Documentation: https://www.amcharts.com/docs/v4/concepts/colors/. It also discusses gradients, etc.