function mixTwoRgba (colors, range = 50) {
  let arr = [], i = 0
  for (let i = 0; i < 4; i++) {
    let round = (i == 3) ? (x) => x : Math.round
    arr[i] = round(
      (colors[0][i] + (
        (colors[1][i] - colors[0][i]) * range / 100
      ))
    )
  }
  return `rgba(${arr})`
}

console.log(
  mixTwoRgba([[234, 126, 53, 0.5], [115, 172, 77, 0.2]], 75)
)