I have always wondered if there's any special significance attached
to colors of passports covers e.g. black for the U.S. (though I've
seen dark green ones), red or black for Japan etc.

Is there?

Hi

The short answer to your question is YES there is special significance
to different colors of passports.  The colors vary between countries
allowing for quick identification of nationality. Also, colors vary
within countries depending on the passport holder's status.

Below are some sample links describing the colors and roles of various
passports ---


KATHMANDU
The red passport is issued ‘only to government officials’ visiting
abroad on a diplomatic mission or an important programme. There are at
least 5,000 such passport holders who have been found misusing them, a
government official said Monday on condition of anonymity.

http://www.nepalnews.com.np/contents/englishdaily/ktmpost/2002/jun/jun11/index1.htm

KODIAK ISLAND BOUROUGH
OFFICIAL PASSPORT  (Red Passport - NO FEE)
http://www.kib.co.kodiak.ak.us/clerk/passport/


GERMANY
Civilian employees of the U.S. and family members are not exempt from
passport requirements.   .. DOD civilian employees and family members
who do not possess either the official (red) passport or a "no fee"
passport must have a tourist passport when traveling between
countries.

http://www.hqusareur.army.mil/htmlinks/Press_Releases/2001/July/20010702-2.htm

If grantee is already a holder of an official (red) passport, he/she
shall submit it to the DFA for revalidation.
http://www.lga.dilg.gov.ph/programs/scholarships2.htm

UK
If you had a red passport, you barely had to stop.  I fell under
"other nationals" with my blue US passport and stood in the short line
to speak with UK immigration.
http://themachman.garynuman.info/weekender1.htm

THE HISTORY OF PASSPORTS IN CANADA
"That same year, the Canadian Citizenship Act, which stipulates that
only Canadian citizens are eligible for a Canadian passport, came into
effect. The familiar blue passport booklet with pale pink pages
similar to the booklets with blue pages issued to British subjects
appeared at that time."
http://www.dfait-maeci.gc.ca/passport/history_e.asp

RUSSIA
" Sergei Zaletin, the commander of the last mission to Mir, had also
been slated to go on this mission, but was unable to because of
bureaucratic difficulties. Russian citizens have three types of
passports: green (diplomatic), blue (business) and red (regular).
Cosmonaut Zaletin, like all members of the Russian military, has a
blue passport. Blue passport holders can only get visas through the
Russian Ministry of Foreign Affairs -- a lengthy and complicated
process."
http://www.space.com/missionlaunches/missions/mir_trackers_010313.html


ECUADOR
13.4. Renewal of Blue Passport for Refugees and Persons without
Nationality
http://www.ecuador.org/ingles/consular/fees.htm



For further information, follow my search strategy for other colors
besides red and blue.

US  "BLUE passport"
US "Red passport"

Note: I found that if I used a country abbreviation (UK or UK) in my
search terms I got better results. There are too many products called
"Passport" that come in various colors.  :-):

Thanks for an interesting question --

--K~

thank you for your answer. thanks to others who added comments.

Hi zhiwenchong

I haven't done any formal research on this one way or another, so I'm
just jotting a bit of information down here about the United Kingdom
passport, as a comment.

The UK passport used to be a dark blue colour until 1988, when as a
result of two things, the International Civil Aviation Authority's new
international standard for machine-readable passports, and the
decision of the EU to introduce a common passport format for its
member states, the UK passport was changed to a purpley-red colour.
The change in colour had no bearing on the status of the passport
itself (it's still a British passport, not a European one), so I
suspect that globally passport colours are chosen merely for local
reasons, and not as part of some grand significant plan.

A full history of the British passport can be found at
http://www.ukpa.gov.uk/textonly/english/t_history_early.htm (and links
from that page).

Regards
iaint-ga

Google search criteria used:
UK passport blue red

My country issues passports of (at least) three different colours: 
Ordinary passports are red, diplomatic passports are black, and
immigrant "passports" (which aren't actually passports, but identity
cards) have a blue colour.

My U.S. passport is dark blue, for whatever that's worth.

My US Passport is dark green and I don't know why?