// An array of discrete values (possibly dynamically generated)
var discreteValues = ['John', 'Jack', 'Jill', 'Jane', 'Jim', 'Joan'];
// 1st scale establishes relationship between continuous [0,1] and
// the array index of the discrete values
var preColor = d3.scaleLinear()
  .domain([0, discreteValues.length - 1])
  .range([0, 1]);
console.log(preColor(discreteValues.indexOf('John'))) // 0
console.log(preColor(discreteValues.indexOf('Jack'))) // 0.2
console.log(preColor(discreteValues.indexOf('Joan'))) // 1.0
// 2nd scale relates a discrete entity name index to a color via the 1st
// scale. We get a continuous value from the entity index going into the 1st
// scale, that continuous value is then mapped to a color using the 2nd scale
var color = i => { return d3.scaleSequential(d3.interpolateRainbow)(preColor(i)); };
console.log(color(discreteValues.indexOf('John'))) // rgb(110, 64, 170)