D3 Fundamentals - Colouring Bar Chart

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <title>Drawing SVG Shapes with D3</title>
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <script>
    var w = 300;
    var h = 100;
    var padding = 2;
    var dataset = [5, 10, 14, 20, 25];
    
    var svg = d3.select("body").append("svg")
    						.attr("width", w)
    						.attr("height", h);
    
    // function to add a specific color/KPI indicator
    function colorPicker(v){					// v as value
    	if(v <= 20) { return "#666666" }
      else if (v > 20) { return "#FF0033"}
    }
    
    // Build bar chart
    svg.selectAll("rect")							// Take all the rect and create new ones
    	.data(dataset)									// for all the diff elements in dataset
    	.enter()												// If there aren't any on the page, create them
      .append("rect")									// Append rect
    		.attr("x", function(d, i) {return i * (w / dataset.length);})
        .attr("y", function(d) { return h - (d * 4); })
    		.attr("width", w / dataset.length - padding)
    		.attr("height", function(d) { return d * 4;	})
    	// Colour using data
    		.attr("fill", function(d) { return "rgb(0, 0," + (d * 10) + ")";})
    	// Colour using the function above
    		.attr("fill", function(d) { return colorPicker(d); })

</script>
</body>

https://d3js.org/d3.v4.min.js