I know this is old, but while I was searching in the support I sen your request and thought I’d help out. you can use this. I also added Opacity since full grayscale is very dark. you can also even include animations, like slowly fading into color mode or grayscale, etc.
.jcarousel-wrapper li img { -webkit-filter: grayscale(1); filter: grayscale(1); opacity: .5;}
.jcarousel-wrapper li img:hover, .wrapper .jcarousel-wrapper li img:focus { -webkit-filter: grayscale(0); filter: grayscale(0); opacity: 1;}
