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>
console
new Vue({
    el:"#app",
    data:{
        attachRed:false,
        color:'green'
    },
    computed:{
	    divClasses:function(){
	        return {
	            red:this.attachRed,
	            blue:!this.attachRed
            }
        }
    },
    methods:{
        
    }
});
<script src="https://cdn.jsdelivr.net/npm/vue"></script>
<div id="app">
    <div class="demo" 
    @click="attachRed=!attachRed"
    :class="divClasses">
    </div>
    <div class="demo" :class="{red:attachRed}"></div>
    <div class="demo" :class="[color, {red: attachRed}]"></div>
<hr>
<p v-model="color"></p>
</div>
.demo{
    width: 100px;
    height: 100px;
    background-color:gray;
    display: inline-block;
    margin: 10px;
}
.red{
    background-color:red;
}
.green{
    background-color:green;
}
.blue{
    background-color:blue;
}