.box {
    max-width: 400px;
    max-height: 400px;
    overflow: hidden;
    cursor: pointer;
}

.box1 {
    background-color: red;
}

.box2 {
    background-color: white;
}

.box3 {
    background-color: green;
}

.box4 {
    background-color: purple;
}

.box5 {
    background-color: yellow;
}

.box6 {
    background-color: orange;
}

.OverlayImg {
    /*width: 100%;
    max-width: 420px;*/
    /*height: auto;*/
    /*opacity: 0.75;*/
    /*overflow: hidden;*/
    /*-webkit-filter: grayscale(100%);
    filter: grayscale(100%);*/
    transition-timing-function: ease-in;
    transition: 1.5s;
}

.OverlayImg:hover {
    opacity: 1;
    -webkit-filter: grayscale(0%);
    filter: grayscale(0%);
    -moz-transform: scale(1.25);
    -webkit-transform: scale(1.25);
    transform: scale(1.25);
    transition-timing-function: ease-in;
    transition: 1.5s;
}