index.css

.block__color {
    width: 500px;
    height: 500px;
    border: 1px solid #000;
    margin: auto;
    margin-bottom: 50px;
    border-radius: 20px;
    transition: background-color .5s ease;
}

.block {
    display: flex;
    justify-content: center;
    flex-direction: column;
    margin-top: 150px;
}

#btn {
    width: 300px;
    margin: auto;
    padding: 15px;
    border-radius: 15px;
    border: 1px solid #000;
    font-weight: 700;
    font-size: 20px;
    transition: all 0.5s ease;
    background-color: #fff;
}

#btn:hover {
    background-color: rgb(11, 228, 11);
    cursor: pointer;
}

.block__hex-color {
    text-align: center;
    display: block;
    margin-bottom: 50px;
    font-size: 20px;
    font-weight: 700;
}

@media (min-width: 320px) {
    .block__color {
        width: 300px;
        height: 300px;
    }
}

@media (min-width: 998px) {
    .block__color {
        width: 500px;
        height: 500px;
    }
}