встроенные функции

$color: #abd3ff;
.col {

  color: darken($color,80%);//затемнить
  color: lighten($color,1%);//осветлить
  transition: border-bottom 1s;
  border-bottom: 1px solid transparentize(red, 1);//rgba(,,,1)
  &:hover{
    border-bottom: 1px solid transparentize(red, .5);//rgba(,,,0.5)
  }
}

.col {
  color: #000912;
  color: #b0d6ff;
  -webkit-transition: border-bottom 1s;
  transition: border-bottom 1s;
  border-bottom: 1px solid rgba(255, 0, 0, 0); }
  .col:hover {
    border-bottom: 1px solid rgba(255, 0, 0, 0.5); }