JSFiddle - React, Tailwind, and code Playground

HTML

<form onsubmit="allow(event)">
<select name="color" id="color">
  <option value="blue">blue</option>
  <option value="green">green</option>
</select>
  <div class="box"></div>
<button>allow</button>
<button>send</button>
</form>

CSS

.box {
  background:black;
  width:50px;height:50px;
}

JavaScript

function send(event) {
    		event.preventDefault ? event.preventDefault() : event.returnValue = false;
        alert(event.target[0].value);
}
        
        
function allow(event) {
    		event.preventDefault ? event.preventDefault() : event.returnValue = false;
        document.querySelector(".box").style.backgroundColor = event.target[0].value;
}