img {
filter: grayscale(100%);
transition: all .5s;
}
img:hover{

filter: grayscale(0%);
}

class vererek yapmanız daha mantıklı


.gray-c {
filter: grayscale(100%);
transition: all .5s;
}
.gray-c:hover{

filter: grayscale(0%);
}