Update color docs.

[anntzer]

Update and sync the color format docs between the colors tutorial and the colors module.
Deprecate plt.colors(), which does nothing other than reproducing this documentation but is not even listed (AFAICT) in the pyplot docs or anywhere else anymore.

As a side note, the tutorials should probably all be listed in the overview (http://matplotlib.org/devdocs/contents.html#overview) of the docs, perhaps in the user guide section.

Feel free to (force-)push edits.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[tacaswell]

The test failures look unrelated.