refactor: rename `colorful` kwarg to `color` (#25292)
refactor: rename `colorful` kwarg to `color` (#25292)

`compare_specs()` had a `colorful` keyword argument, but everything else in
spack uses `color` for this.

- [x] rename the argument
- [x] make the default follow spack's `--color=always/never/auto` setting
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack