<p>Altere a cor de fundo na lista de seleção e veja a cor da caixa mudar</p>
<select id="cor">
  <option value="#000000">Preto</option>
  <option value="#FF0000">Vermelho</option>
  <option value="#00FF00">Verde</option>
  <option value="#0000FF">Azul</option>
</select>
<div id="caixa-colorida"></div>
div {
  width: 350px;
  height: 150px;
  background: black;
  margin-top: 20px;
}
document.getElementById("cor").onchange = () => {
  let cor = document.getElementById("cor").value;
  
  document.getElementById("caixa-colorida").style.background = cor;
}

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