gradient.js

function lerp(a, b, t) {
  return a + t * (b - a);
}

function  mix(a, b, t) {
  return a.map((v, i) => lerp(v, b[i], t));
}

function clamp(x, min, max) {
  if (x < min) { return min; }
  if (x > max) { return max; }
  return x;
}

function gradient(colors, x) {
  x = clamp(x, 0.0, 1.0);

  const numColors = colors.length - 1;
  const index = Math.floor(numColors * x);
  const length = 1.0 / numColors;

  const color1 = colors[index];
  const color2 = colors[index + 1] || color1;

  return mix(color1, color2, (x - index * length) * numColors);
}

const colors = [
  [0xff, 0x00, 0x00],
  [0xff, 0xcc, 0x00],
  [0x00, 0xff, 0x00],
  [0x00, 0xff, 0xcc],
  [0x00, 0x00, 0xff],
  [0xff, 0x00, 0x00]
];

for (let i = 0; i < 100; i++) {
  const style = `background: rgb(${gradient(colors, i / 100.0).map(Math.floor).join()}`;
  console.log("%c                       ", style);
}