score:1

Accepted answer

Given your desired outcome...

the idea then is to give a greener color (meaning it is more important), from a red to green color scale, to the skill that has higher percentage.

..., an ordinal scale is not the adequate choice.

You can use a linear scale with two colours in the range, "red" and "green", but the result is not nice. So, this solution uses three colours in the range: "red", "yellow" and "green".

The trick to do that is using three values in the domain:

var color = d3.scale.linear()
    .domain([d3.min(pie_data, d => d.course_area_percentage),
        d3.mean(pie_data, d => d.course_area_percentage),
        d3.max(pie_data, d => d.course_area_percentage)
    ])
    .range(["red", "yellow", "green"]);

Here is a demo:

var pie_data = [{
  "hard_skill": "Strategy",
  "course_area_percentage": 9
}, {
  "hard_skill": "Analytics",
  "course_area_percentage": 18
}, {
  "hard_skill": "Economics",
  "course_area_percentage": 11
}, {
  "hard_skill": "Finance",
  "course_area_percentage": 7
}, {
  "hard_skill": "HR",
  "course_area_percentage": 5
}, {
  "hard_skill": "Innovation",
  "course_area_percentage": 2
}, {
  "hard_skill": "International",
  "course_area_percentage": 5
}, {
  "hard_skill": "Marketing",
  "course_area_percentage": 7
}, {
  "hard_skill": "Operations",
  "course_area_percentage": 14
}, {
  "hard_skill": "Others",
  "course_area_percentage": 11
}, {
  "hard_skill": "Project",
  "course_area_percentage": 11
}]
var width = 500,
  height = 500,
  radius = Math.min(width, height) / 2;


var color = d3.scale.linear()
	.domain([d3.min(pie_data, d=>d.course_area_percentage),
	d3.mean(pie_data, d=>d.course_area_percentage),
	d3.max(pie_data, d=>d.course_area_percentage)])
	.range(["red", "yellow", "green"]);

var arc = d3.svg.arc()
  .outerRadius(radius - 10)
  .innerRadius(0);

var pie = d3.layout.pie()
  .sort(null)
  .value(function(d) {
    return d.course_area_percentage;
  });

var svg = d3.select("body").append("svg")
  .attr("width", width)
  .attr("height", height)
  .append("g")
  .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")");

var g = svg.selectAll(".arc")
  .data(pie(pie_data))
  .enter().append("g")
  .attr("class", "arc");

g.append("path")
  .attr("d", arc)
  .style("stroke", "gray")
  .style("fill", function(d) {
    return color(d.data.course_area_percentage);
  });

g.append("text")
.attr("text-anchor", "middle")
  .attr("transform", function(d) {
    return "translate(" + arc.centroid(d) + ")";
  })
  .attr("dy", ".35em")
  .text(function(d) {
    return d.data.hard_skill;
  });

function type(d) {
  d.population = +d.population;
  return d;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"></script>

As an alternative solution, if you want to use the color array you wrote in the question...

.range(["#008e15", "#3fe256", "#eef200", "#f19d00", "#d7191c"]);

... you should use a quantize (or quantile) scale instead:

var color = d3.scale.quantize()
    .domain(d3.extent(pie_data, d => d.course_area_percentage))
    .range(["#d7191c", "#f19d00", "#eef200", "#3fe256", "#008e15"]);

Here is the demo:

var pie_data = [{
  "hard_skill": "Strategy",
  "course_area_percentage": 9
}, {
  "hard_skill": "Analytics",
  "course_area_percentage": 18
}, {
  "hard_skill": "Economics",
  "course_area_percentage": 11
}, {
  "hard_skill": "Finance",
  "course_area_percentage": 7
}, {
  "hard_skill": "HR",
  "course_area_percentage": 5
}, {
  "hard_skill": "Innovation",
  "course_area_percentage": 2
}, {
  "hard_skill": "International",
  "course_area_percentage": 5
}, {
  "hard_skill": "Marketing",
  "course_area_percentage": 7
}, {
  "hard_skill": "Operations",
  "course_area_percentage": 14
}, {
  "hard_skill": "Others",
  "course_area_percentage": 11
}, {
  "hard_skill": "Project",
  "course_area_percentage": 11
}]
var width = 500,
  height = 500,
  radius = Math.min(width, height) / 2;


var color = d3.scale.quantize()
	.domain(d3.extent(pie_data, d=>d.course_area_percentage))
	.range(["#d7191c", "#f19d00", "#eef200", "#3fe256", "#008e15"]);

var arc = d3.svg.arc()
  .outerRadius(radius - 10)
  .innerRadius(0);

var pie = d3.layout.pie()
  .sort(null)
  .value(function(d) {
    return d.course_area_percentage;
  });

var svg = d3.select("body").append("svg")
  .attr("width", width)
  .attr("height", height)
  .append("g")
  .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")");

var g = svg.selectAll(".arc")
  .data(pie(pie_data))
  .enter().append("g")
  .attr("class", "arc");

g.append("path")
  .attr("d", arc)
	.style("stroke", "gray")
  .style("fill", function(d) {
    return color(d.data.course_area_percentage);
  });

g.append("text")
.attr("text-anchor", "middle")
  .attr("transform", function(d) {
    return "translate(" + arc.centroid(d) + ")";
  })
  .attr("dy", ".35em")
  .text(function(d) {
    return d.data.hard_skill;
  });

function type(d) {
  d.population = +d.population;
  return d;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"></script>


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