$ <<. cat >old.csv
color,count
red,3467
orange,670
yellow,27
blue,6944
.

$ <<. cat >new.csv
color,count
red,3472
blue,6838
orange,694
purple,12
.

$ # with --lp, --rp
   <new.csv mlr --c2p \
      join -f old.csv --ul --ur -j color --lp=old_ --rp=new_ \
      + unsparsify
color  old_count new_count count
red    3467      3472      -
blue   6944      6838      -
orange 670       694       -
purple -         -         12
yellow -         -         27

$ # vs with `rename` on both files
   <new.csv mlr --c2p \
      rename count,new_count \
      + join -f <(mlr -c rename count,old_count old.csv) --ul --ur -j color \
      + unsparsify
color  old_count new_count
red    3467      3472
blue   6944      6838
orange 670       694
purple -         12
yellow 27        -