SOURCE

const domItem = document.querySelectorAll('.item');

function toggleColor(e) {
    const color = '#'+ Math.floor(Math.random() * 0xffffff).toString(16).padEnd(6, '0');
    e.target.style.backgroundColor = color;
}

for(let i = 0; i < domItem.length; i++) {
    domItem[i].addEventListener('mouseover', toggleColor);
}

<div class="list">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
</div>

.list {
    width: 400px;
}

.item {
    float: left;
    width: 100px;
    height: 100px;
    background: #09f;
}