wxml:

 <input type="text" placeholder="请输入您的答案" 
            placeholder-style="animation: input-ani  0.6s forwards;" value="{{value}}"/>

css:

@keyframes input-ani {
    0% {
        color:#FDF2CE;
    }
    99%{
        color:#FDF2CE; 
    }
    100% {
        color: #242424
    }
}