.img:hover {
  transition: filter .5s ease-in-out;
  -webkit-filter: grayscale(0%); /* Ch 23+, Saf 6.0+, BB 10.0+ */
  filter: grayscale(0%); /* FF 35+ */
}
.img {
  -webkit-filter: grayscale(100%); /* Ch 23+, Saf 6.0+, BB 10.0+ */
  filter: grayscale(100%); /* FF 35+ */
}