body {
    background-color: gray;
}
h1 {
    color: red;
    text-align: center;
}
.cool {
    visibility: hidden;
    width: 100px;
    height: 100px;
    background-color: green;
    transition: background-color 1s;
}
.cool.red{
    visibility: visible;
    background-color: red;
}