learnin-some-vue/app.js

var app = new Vue({
  el: '#app',
  data: {
    attachRed: false,
    attachGreen: false,
    attachBlue: true
  },
  computed:{
    divClasses(){
      return {
        red: this.attachRed,
        blue: !this.attachRed
      }
    }
  }
});