/*.rectangle {
    width: 400px;
    height: 300px;
    border-radius: 10px;
    transition: background-color 0.8s;
}*/

.blue {
    width: 400px;
    height: 300px;
    border-radius: 10px;
    transition: background-color 0.8s;
    background-color: royalblue;
}

.red {
    width: 400px;
    height: 300px;
    border-radius: 10px;
    transition: background-color 0.8s;
    background-color: firebrick;
}


.green {
    width: 400px;
    height: 300px;
    border-radius: 10px;
    transition: background-color 0.8s;
    background-color: forestgreen;
}