[SOLVED] How to create smoothed line instead of sharp line in amcharts 4?

You cannot use tensionX, tensionY or smoothing with propertyFields. You have to create axis ranges instead. Take a look at the following issue on GitHub:

The smoothed curve algorithm works by bending the path around multiple points.

Now, when you are using propertyFields to color specific section of the line, it creates a separate path for each separately-colored segment. If that segment encompasses only two points, the smoothing algorithm does not have anything to work with, hence the straight line.

If you remove series.propertyFields.stroke = "color"; and replace it with series.smoothing = "monotoneX";, you could do something like this:

function colorize(startDate, endDate, color) {
  var range = dateAxis.createSeriesRange(series);
  
  range.date = new Date(startDate + ' 12:00');
  range.endDate = new Date(endDate + ' 12:00');
  range.contents.stroke = am4core.color(color); 
}

for (var i = 0; i < chart.data.length; i++) {
  var startDate = chart.data[i].date,
      endDate = (i < chart.data.length - 1) && chart.data[i + 1].date,
      color = chart.data[i].color;
  
  colorize(startDate, endDate, color);
}

Here is your code with these modifications and additions:

/**
 * ---------------------------------------
 * This demo was created using amCharts 4.
 * 
 * For more information visit:
 * https://www.amcharts.com/
 * 
 * Documentation is available at:
 * https://www.amcharts.com/docs/v4/
 * ---------------------------------------
 */

// Themes begin
am4core.useTheme(am4themes_animated);
// Themes end



// Create chart instance
var chart = am4core.create("chartdiv", am4charts.XYChart);

// Add data
chart.data = [{
  "date": "2012-03-01",
  "price": 20,
  "color": "#00FF00"
}, {
  "date": "2012-03-02",
  "price": 75,
  "color": "#FF0000"
}, {
  "date": "2012-03-03",
  "price": 15,
  "color": "#00FF00"
}, {
  "date": "2012-03-04",
  "price": 75,
  "color": "#00FF00"
}, {
  "date": "2012-03-05",
  "price": 158,
  "color": "#FF0000"
}, {
  "date": "2012-03-06",
  "price": 57,
  "color": "#FF0000"
}, {
  "date": "2012-03-07",
  "price": 107,
  "color": "#FF0000"
}, {
  "date": "2012-03-08",
  "price": 89,
  "color": "#FF0000"
}, {
  "date": "2012-03-09",
  "price": 75,
  "color": "#00FF00"
}, {
  "date": "2012-03-10",
  "price": 132,
  "color": "#00FF00"
}, {
  "date": "2012-03-11",
  "price": 380,
  "color": "#FF0000"
}, {
  "date": "2012-03-12",
  "price": 56,
  "color": "#00FF00"
}, {
  "date": "2012-03-13",
  "price": 169,
  "color": "#FF0000"
}, {
  "date": "2012-03-14",
  "price": 24,
  "color": "#00FF00"
}, {
  "date": "2012-03-15",
  "price": 147,
  "color": "#00FF00"
}];

// Create axes
var dateAxis = chart.xAxes.push(new am4charts.DateAxis());
dateAxis.renderer.grid.template.location = 0;
dateAxis.renderer.minGridDistance = 50;

var valueAxis = chart.yAxes.push(new am4charts.ValueAxis());
/* valueAxis.logarithmic = true; */
valueAxis.renderer.minGridDistance = 20;

// Create series
var series = chart.series.push(new am4charts.LineSeries());
series.dataFields.valueY = "price";
series.dataFields.dateX = "date";
series.strokeWidth = 3;
// series.propertyFields.stroke = "color";
series.smoothing = "monotoneX"; // <--- HERE

var bullet = series.bullets.push(new am4charts.CircleBullet());
bullet.circle.fill = am4core.color("#fff");
bullet.circle.strokeWidth = 3;

// Add cursor
chart.cursor = new am4charts.XYCursor();
chart.cursor.fullWidthLineX = true;
chart.cursor.xAxis = dateAxis;
chart.cursor.lineX.strokeWidth = 0;
chart.cursor.lineX.fill = am4core.color("#000");
chart.cursor.lineX.fillOpacity = 0.1;

// Add scrollbar
chart.scrollbarX = new am4core.Scrollbar();

// Add a guide
let range = valueAxis.axisRanges.create();
range.value = 90.4;
range.grid.stroke = am4core.color("#396478");
range.grid.strokeWidth = 1;
range.grid.strokeOpacity = 1;
range.grid.strokeDasharray = "3,3";
range.label.inside = true;
range.label.text = "Average";
range.label.fill = range.grid.stroke;
range.label.verticalCenter = "bottom";

// ========================= HERE =========================

function colorize(startDate, endDate, color) {
  var range = dateAxis.createSeriesRange(series);

  range.date = new Date(startDate + " 12:00");
  range.endDate = new Date(endDate + " 12:00");
  range.contents.stroke = am4core.color(color); 
}

for (var i = 0; i < chart.data.length; i++) {
  var startDate = chart.data[i].date,
      endDate = (i < chart.data.length - 1) && chart.data[i + 1].date,
      color = chart.data[i].color;

  colorize(startDate, endDate, color);
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
}

#chartdiv {
  width: 100%;
  height: 500px;
}

<script src="https://cdn.amcharts.com/lib/4/core.js"></script>
<script src="https://cdn.amcharts.com/lib/4/charts.js"></script>
<script src="https://cdn.amcharts.com/lib/4/themes/animated.js"></script>
<div id="chartdiv"></div>