I use the "grep with color for lines plus the empty string" so frequently that I have a function for it:

  function highlight() {
    local args=( "$@" )
    for (( i=0; i<${#args[@]}; i++ )); do
      if [ "${args[$i]:0:1}" != "-" ]; then
        args[$i]="(${args[$i]})|$"
        break
      fi
    done
    grep --color -E "${args[@]}"
  }

This is only to be used as a filter, since it mangles filenames.

I'm curious, does ack support a highlight-only mode?