Before I knew anything about printing, I thought duotones were
actually 2-color separations (that is, as close to the actual
color as you could get with just two inks). That's not the way
it's done now, but at one time 2-color printing was considered a
serious alternative to 3- and 4-color printing. The first three
Technicolor processes were two-color, but they were not
successful and were replaced by a three-color process in 1932.

2-color separations never caught on because they're too dependent
on the content of the image. Ironically, it would be easier now
with scanners and computers, but 4-color printing is so common
that it's probably not worth it anymore.

I knew that Hexachrome wasn't the first 6-color printing system,
but I was surprised at how common 6-color litho was up until the
1950s-- so common that Sipley felt it necessary to remind his
readers (in 1951) that 4-color litho was also commonly used.
I assume 6-color litho died out because color correction methods
had become good enough that they didn't need the extra colors
anymore.

[See part 1 for bibliography]

****Two Color Printing****

[Coote, p75]
[describing Ives' two-color "Polychrome" system]
"From the orange-red record negative an iron-toned (blue-green)
print was made using Defender Ivora, a bromide emulsion coated on
a white pigmented acetate base. To this image was cemented a
red/magenta positive printed from the blue-green record negative
on to Kodak Wash-Off-Relief film. The gelatine relief image was
dyed with a mixture of orange and magenta dyes which tended to
produce yellowish highlights that favored flesh colours and
shadows that remained reasonably neutral."

[Sipley, p84]
"An interest in two-color printing was manifested by
photoengravers and gravure workers. In June of 1929 *The Inland
Printer* printed a picture of red cherries in both three colors
and in two colors. In one case the standard red, yellow, and blue
were used and in the other orange and green. The originals came
from *The Bridewell and Bromley Magazine*, house publication of
the Grout Engraving Company of London, England. The results were
amazing, and the Grout company suggested that where economy was
necessary the two-color idea was far better than using
three-color plates with cheaper inks on low-priced and inferior
paper. The Sungravure company in England turned out some fine
examples of two-color gravure work around 1930. In America the
Beck Engraving Company have produced some extraordinarily fine
examples of two-color gravure. In the main however most two-color
printing has been to give the effect of an added color rather
than a multi-color picture. The failure to utilize to the full
the multi-color possibilities to be realized through the use of
two colors must not be entirely charged against the photographic
and photo-mechanical fields. A great deal of education is needed
in commercial art schools, as much by the faculties as by the
students, and in the art departments of advertising agencies, if
the fullest potential of the two-color process is to be placed at
the service of consumers of photography and printed
reproduction."

[Hunt, p137-8]
"If all the colours of a scene could be matched by a mixture of
only two dyes, colorimetrically correct two-color subtractive
reproductions could be obtained. The colours in most scenes,
howver, are not confined even approximately to the above type of
restrictions, but two-colour (usually cyan and orange)
reproductions can sometimes be surprisingly realistic, and in
cinematography have been used commercially (Cornwell-Clyne, 1951,
p343)."

"The acceptability of two-colour reproductions is markedly
dependant on the subject matter. Indoor scenes are often very
realistic, probably because light sources very deficient in blue
content, such as candles and yellowish tungsten lamps, are
commonly experienced, and the low level of the blue signal tends
to reduce vision to nearly two variables. Outdoor scenes, on the
other hand, are generally less acceptable, and the inability to
render the hue difference between blue sky and green foliage is a
serious drawback."

****Extra Inks****

[Yule, p303-304]
"Until recently, the use of additional colors was common
practice, especially in lithography. For example, a pink and a
light cyan has often been used in addition to a deep red or
magenta and a deep blue or cyan. This improves the purity of
light blues, pinks, and flesh tones, which are usually degraded
because of proportionality failure (Chapter 9) associated with
the halftone structure."

"A number of special sets of four-color or five-color inks have
been proposed. Zander (1924) patented a four-color system based
on the use of green, magenta, blue, and yellow inks. Jacobs
(1924) proposed the use of a fourth ink similar in color to
Prussian Blue. Murray (1934) proposed the addition of a purple or
lavender ink. With these systems, there is considerable
overlapping of the absorption bands."

"Friedman (1944, p32) suggested splitting the spectrum in to four
instead of three equal bands, and Ball (1950a) adjusted the bands
to take advantage of what is known about color vision and color
mixture. Work with Ball's system has been described by Leekley,
Cox, and Jordan (1953). Ball suggested several sets of inks, each
set consisting of a yellow, a pink, a purple, and a cyan. His
compromise set is intended to split the spectrum at 485, 545, and
600 mu. The purple ink absorbs what he calls the 'chlor' band, a
greenish yellow band extending from 545 to 600 mu. Some
overlapping of absorption bands is of course unavoidable with
available pigments. A masking method was used to compensate for
this, but a scanner could probably do this more effectively. Ball
(1950b) also suggested a five-color set which covered an even
larger gamut of colors."

{Sipley, p133]
"Although the previous description has been entirely devoted to
six-color photolithography, it must not be assumed that all
photolithography is done by the six-color method. The use of four
colors in offset printing was successfully employed by Huebner as
early as 1910 (see black-and-white reproduction on page 50) and
today some of the finest color printing in the country is being
done by four-color photolithography."

****Printing on Two Presses At Once[!]****

[Sipley, p129]
"Modern color presses are built in the form of two-color and
four-color units, although the latter are extremely expensive and
consequently few in number. Two-color presses prove to be very
flexible for four-color and six-color work whether in letterpress
(although more than four colors is a rarity) or in
photolithography. Large printing establishments fitted with
two-color presses find it possible to run a four-color job
complete at one time by two sets of plates on one press and the
other two sets of plates on the second press and feeding the
sheets from the first press right through the second press."

"In this kind of setup it is practical to run a four-color job
through two presses, feeding from one press directly to the
other."

"A battery of two-color presses similar to that shown on page 135
makes it possible to run a six-color series though in the same
manner in which two two-color letterpress units handle a
four-color job."