For the examples below, you can use xs, sm, md, lg and xl as the mixin parameter. I have used md.
/* Breakpoints lower than */
@include media-breakpoint-down(md) {
color: red;
}
/* Breakpoints higher than */
@include media-breakpoint-up(md) {
color: blue;
}
/* Exact breakpoint */
@include media-breakpoint-only(md) {
color: green;
}