Read colorization from environment variable, if command line is not set (#23130)

lib/spack/docs/basic_usage.rst

@@ -27,12 +27,18 @@ It is recommended that the following be put in your ``.bashrc`` file:
 
 If you do not see colorized output when using ``less -R`` it is because color
 is being disabled in the piped output. In this case, tell spack to force
-colorized output.
+colorized output with a flag
 
 .. code-block:: console
 
     $ spack --color always | less -R
 
+or an environment variable
+
+.. code-block:: console
+
+   $ SPACK_COLOR=always spack | less -R
+
 --------------------------
 Listing available packages
 --------------------------

lib/spack/spack/main.py

@@ -354,7 +354,8 @@ def make_argument_parser(**kwargs):
         dest='help', action='store_const', const='long', default=None,
         help="show help for all commands (same as spack help --all)")
     parser.add_argument(
-        '--color', action='store', default='auto',
+        '--color', action='store',
+        default=os.environ.get('SPACK_COLOR', 'auto'),
         choices=('always', 'never', 'auto'),
         help="when to colorize output (default: auto)")
     parser.add_argument(