Enter the style below into the ‘Custom CSS’ field and adjust them:
/* portfolio photo hover color */
.fusion-image-wrapper .fusion-rollover {
background-image: linear-gradient(top, rgba(224,224,224,0.8) 0%, #333 100%);
background-image: -webkit-gradient(linear, left top, left bottom, color-stop(0, rgba(224,224,224,0.8)), color-stop(1, #333));
}