Lib\idlelib\colorizer.py, line 25, is:
  stringprefix = r"(?i:\br|u|f|fr|rf|b|br|rb)?"

The r prefix, but only the r prefix, must be preceded by a non-word character.

On pydev thread "IDLE colorizer", MRAB noted: "The \b will apply only to the first choice. Shouldn't it be more like:
  stringprefix = r"(?:\b(?i:r|u|f|fr|rf|b|br|rb))?"

I think the \b should be removed instead.  Currently uf'a' gets f'a' colored, while ur'a' only gets  'a' colored.  (Tim Peters pointed out the difference with a different example.) I prefer coloring the maximal legal string rather than the minimum.    I think the contrast between two chars legal by themselves, but differently colored when put together, makes the bug more obvious.

The current test is part of the htest.  The section
# All valid prefixes for unicode and byte strings should be colored
contains "ur'is invalid'".  This should be moved to a new section with other invalid combinations.

# Invalid combinations of legal characters should be half colored.
ur'x', ru'x', uf'x', fu'x', UR'x'
(only one CAPS example is needed now).

Sounds good (removing \b) to me, Terry!

To see the visually verified test, run Lib/idlelib/colorizer as the main module, either from a command line or editor, and click the button.

New changeset da58533ac67b01ce8f6466e6f03ff6b8b3bb04d5 by Terry Jan Reedy in branch 'master':
bpo-33204: IDLE: consistently color invalid string prefixes (#6344)
https://github.com/python/cpython/commit/da58533ac67b01ce8f6466e6f03ff6b8b3bb04d5

New changeset 6130675efeb3c23cebc43ab07c49eb62cd03cacf by Miss Islington (bot) in branch '3.7':
bpo-33204: IDLE: consistently color invalid string prefixes (GH-6344)
https://github.com/python/cpython/commit/6130675efeb3c23cebc43ab07c49eb62cd03cacf

New changeset 16cf84b4fbe78f9d876e0335f33459f1b92b7bf0 by Miss Islington (bot) in branch '3.6':
bpo-33204: IDLE: consistently color invalid string prefixes (GH-6344)
https://github.com/python/cpython/commit/16cf84b4fbe78f9d876e0335f33459f1b92b7bf0

Is ''or'' correctly colorized?

Yes, still correct, as a keyword.  Legal "x or''" is colored as "x" <normal> "or" <keyword> "''" <string>.  Legal "if'':" is colored as "if" <keyword> "''" <string> ":" <normal>".  New PR revises colorizer test and add the above.

New changeset 55966f3a0d5f1bf823bd22ce1abbde267b06552f by Terry Jan Reedy in branch 'master':
bpo-33204: IDLE - revise and extend colorizer test. (GH-6347)
https://github.com/python/cpython/commit/55966f3a0d5f1bf823bd22ce1abbde267b06552f

New changeset fa91aff07c16ad48a63580c2909934be92a5e9c8 by Miss Islington (bot) in branch '3.7':
bpo-33204: IDLE - revise and extend colorizer test. (GH-6347)
https://github.com/python/cpython/commit/fa91aff07c16ad48a63580c2909934be92a5e9c8

New changeset 306559e6ca15b86eb230609f484f48132b7ca383 by Miss Islington (bot) in branch '3.6':
bpo-33204: IDLE - revise and extend colorizer test. (GH-6347)
https://github.com/python/cpython/commit/306559e6ca15b86eb230609f484f48132b7ca383

Should this be closed as resolved?