Exercise: improve the color selector

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Take the examples/perlarrays/process_command_line.pl script and improve it in several ways:

1. Check if the value given on the command line is indeed one of the possible values and don't let other colors pass.
2. Allow a --force flag that will disregard the previously implemented restriction. Meaning
3. --color Purple should still report error but
4. --color Purple --force should accept this color as well.