SOURCE

const domSelect = document.querySelector('select');
const domBox = document.querySelector('.box');

domSelect.addEventListener('change', setMonth);

function setMonth() {
    let color = domSelect.value;
    
    switch(color){
        case 'red':
            updateColor('white', 'red');
            break;
        case 'green':
            updateColor('white', 'green');
            break;
        case 'blue':
            updateColor('white', 'blue');
            break;
        case 'yellow':
            updateColor('black', 'yellow');
            break;
        case 'orange':
            updateColor('black', 'orange');
            break;
        default:
            updateColor('black', 'white');

    }
}

function updateColor(color, bgColor) {
    domBox.style.color = color;
    domBox.style.backgroundColor = bgColor;
}

<p>
    Select Color：
    <select>
        <option value="">请选择</option>
        <option value="red">red</option>
        <option value="green">green</option>
        <option value="blue">blue</option>
        <option value="yellow">yellow</option>
        <option value="orange">orange</option>
    </select>
</p>
<div class="box">I am a Box!</div>

.box {
    line-height: 100px;
    text-align: center;
    background-color: #fff;
}