Found the source, sort of...

http://www.airweb.faa.gov/Regulatory...rgAdvisoryCirc
ular.nsf/0/c2f903127f0f41e4862569ae006d44d7/$FILE/AC20-88A.pdf

ANNOUNCEMENT OF AVAILABILITY OF ADVISORY CIRCULAR 20-88A; LINES ON THE
MARKING OF AIRCRAFT POWERPLANT INSTRUMENTS (DISPLAYS)

g. Tachometer (Reciprocating Engine Aircraft)
Red Radial - At maximum permissible rotational speed (rpm).
Green Arc - From maximum rotational speed for continuous operation to
minimum recommended for continuous operation (except in the restricted
range, if any). For propeller driven small airplanes which must meet
the requirements in Appendix F to Part 36 of the FAR at maximum normal
operating power, from the corrected rotational speed used to establish
the aircraft noise level to the minimum recommended for continuous
operation (except in the restricted ranges, if any).
Yellow Arc - From maximum rotational speed for continuous operation to
maximum permissible rotational speed.
Yellow Arc (Offset) - Conditional operating range for one engine
inoperative flight that exists above the normal takeoff rating limit
(if applicable).
Red Arc - Range(s) in which operation is restricted, except to pass
through, for all operating conditions because of excessive stresses,
etc.