SOURCE

function getRandomColor() {
    var letters = '0123456789ABCDEF';
    var color = '#';
    for (var i = 0; i < 6; i++) {
        color += letters[Math.floor(Math.random() * 16)];
    }
    return color;
}

const container = document.querySelector('.container')

for (let i = 0; i < 40; i++) {
    const item = document.createElement('div')
    item.style.background = `${getRandomColor()}`
    item.className = 'item'
    container.appendChild(item)
}

<div class="container"></div>

.container {
    width: 300px;
    height: 500px;
    overflow: auto
}
.item {
    width: 100%;
    height: 30px;
    margin: 5px;
}