doccano/frontend/plugins/color.ts

import Vue from 'vue'

declare module 'vue/types/vue' {
  interface Vue {
    $contrastColor(hexString: string): string
  }
}

Vue.prototype.$contrastColor = (hexString: string) => {
  // W3c offers a formula for calculating ideal color:
  // https://www.w3.org/TR/AERT/#color-contrast
  const r = parseInt(hexString.substr(1, 2), 16)
  const g = parseInt(hexString.substr(3, 2), 16)
  const b = parseInt(hexString.substr(5, 2), 16)
  return (r * 299 + g * 587 + b * 114) / 1000 < 128 ? '#ffffff' : '#000000'
}