create a scale from multiple colors

polycolorScale

// mashup
// https://gist.github.com/maxwells/8251275
// https://github.com/andrewbranch/Colorpolate

// convert 6-digit hex to rgb components;
// accepts with or without hash ("335577" or "#335577")
function hexToRgb(hex) {
  const [r, g, b] = hex.replace(/#/,'').match(/.{1,2}/g)
  return [
    parseInt(r, 16),
    parseInt(g, 16),
    parseInt(b, 16)
  ]
}

function interpolate(start, end, amount) {
	return parseInt(end - (end - start) * amount, 10)
}

function polycolorScale(colors, amount) {
	const colorCount = colors.length

  const stops = [0]
  for (let i = 1; i < colorCount; i++) {
		stops[i] = stops[i - 1] + 1 / (colorCount - 1)
	}
  // const stops = [0, 300, 500]

  // convert all colors to rgb
  colors = colors.map(color =>
    /^#[0-9A-F]{6}$/i.test(color) ? hexToRgb(color) : color
  )

  // calculate current scale
	for (let i = 1; i < stops.length; i++) {
    const color = colors[i]
    const lastColor = colors[i - 1]
    const stop = stops[i]
    const laststop = stops[i - 1]
    const newColor = []

    for (let i = 0; i < 3; i++) {
      newColor[i] = interpolate(color[i], lastColor[i], 1 - (stop - amount) / (stop - laststop))
    }

		return newColor
	}
}