Get interpolated color between two other.

interpolateColor.js

const parseHexColor = hexColor => hexColor
  .match(/(?=#)?\w{2}/g)
  .map(c => parseInt(c, 16));

/**
 * @param {string} beginColor eg. #FFFFFF must have 6 chars.
 * @param {*} endColor eg. #000000 must have 6 chars.
 * @param {*} percent must ∈ [0, 1].
 */
const interpolateColor = (beginColor, endColor, percent) => {
  const bColor = parseHexColor(beginColor);
  const eColor = parseHexColor(endColor);
  const output = [...new Array(3)].map((_, ii) =>
    Math.round(bColor[ii] + (eColor[ii] - bColor[ii]) * percent)
      .toString(16)
  )
  return `#${output.join('')}`;
}