d3-ordinal.js

let myData = ['R', 'G', 'B']

let ordinalScale = d3.scaleOrdinal()
  .domain(myData)
  .range(['red', 'green', 'blue']);

ordinalScale('R');    // red
ordinalScale('G');    // green
ordinalScale('B');    // blue
ordinalScale('Apr');  // red