Sass nth-child nesting

I'd be careful about trying to get too clever here. I think it's confusing as it is and using more advanced nth-child parameters will only make it more complicated. As for the background color I'd just set that to a variable.

Here goes what I came up with before I realized trying to be too clever might be a bad thing.

#romtest {
 $bg: #e5e5e5;
 .detailed {
    th {
      &:nth-child(-2n+6) {
        background-color: $bg;
      }
    }
    td {
      &:nth-child(3n), &:nth-child(2), &:nth-child(7) {
        background-color: $bg;
      }
      &.last {
        &:nth-child(-2n+4){
          background-color: $bg;
        }
      }
    }
  }
}

and here is a quick demo: http://codepen.io/anon/pen/BEImD

----EDIT----

Here's another approach to avoid retyping background-color:

#romtest {
  %highlight {
    background-color: #e5e5e5; 
  }
  .detailed {
    th {
      &:nth-child(-2n+6) {
        @extend %highlight;
      }
    }

    td {
      &:nth-child(3n), &:nth-child(2), &:nth-child(7) {
        @extend %highlight;
      }
      &.last {
        &:nth-child(-2n+4){
          @extend %highlight;
        }
      }
    }
  }
}