Creating a Spot Color Channel in Photoshop

Question

Can anyone teach me how to make a Spot Color Channel on Photoshop? I am using this image as an example. The blue color in the background needs to be printed mostly with a spot color that is close to the original painting color (ex: Pantone Blue 072C) . But the CMYK color still should be present in within the blues to resemble the subtle nuances of the brush strokes. I can’t figure it out on my own.
For more samples of painting images, please go to http://www.flickr.com/photos/batzorig
The reason I am doing this is because the artist’s works were represented on many publications over the years from many different countries. And we found out that CMYK color alone cannot duplicate the blue color very well (I mean the blue only, no problem with other colors). I know, it is hard to believe, but if you see all the publications and compare it to the original art, you would see the huge difference in color saturation and luminosity. That is the reason, I think it is necessary to use a spot blue for an upcoming catalog of the artist.
It will be a huge help in my work progress if I learn to make the Spot Color Channel. Then I can talk with the printers about this method.

Answer 1

Understanding color space diagrams for CIE(1931) and CIELab isn't easy,
but gradually one gets used to interpret them by intuition.
(1) helps understanding CIE(1931) chromaticity.
The horseshoe contour contains all possible colors with luminance left
out. Indicated colors are just for orientation. Yellow is the 'locus' for
bright yellow  and brown (dark yellow) as well. White is the locus for
white and black - colors without saturation at a certain center. For two
colors on the same ray to the center, the color with larger distance is
more brilliant or vibrant.
You're right - adding green and orange ink in Hexachrome doesn't
extend the gamut at the blue side. But Cyan and Magenta (and Yellow
and Black) are different to common CMYK inks as well, and this
delivers more brilliant blues.
(2) shows the effect, now in CIELab in a horizontal slice for constant
lightness. Besides theoretical aspects, the diagram can be (again) inter-
preted by intuition.
The complete diagram is threedimensional. A shown color is not just
a placeholder (as in the CIE chromaticity diagram) but a more or less
correct reproduction, as good as possible, depending on the medium.
It seems indeed that there are few print houses using Hexachrome,
but (3)  is one of them.
What's to do for the actual catalog? Printing by inkjet would be a
solution, because inkjets can use additional inks like Green, Orange,
Blue, mostly by replacing standard inks like LightCyan, LightMagenta,
Gray. The inkjet can be calibrated by GretagMacbeth ProfileMaker's
Multicolor Module. That would be very expensive. Even without
additional inks the blues can be reproduced fairly good.
As already said - just adding a Spot Blue would cure the problem
for some paintings, but there is no systematical workflow.
If Hexachrome is not an option, then one may try to modify the
blue in the image by shifting it towards cyan with less lightness,
which is better printable.
Of course wrong, but what finally counts is the impression.
A friend of mine is a famous German photographer for calendars
and tourist guide books which contain plenty images with blue skies.
So far he got almost always pleasant print results - by applying
appropriate image processing with Soft Proofing in Photoshop.
Examples are in (4).
It would be nice, if somebody who is practically working with
Hexachrome or other Multicolor processes could contribute.
About this question:
I don't know if it is about LAB colors or Hexachrome colors.
The CIE (1931) color space appears mainly by two representations:
1. CIE xyY Chromaticity diagram (horseshoe)
2. CIELab = Lab
These are color spaces which contain all possible colors. Spot inks
and primary inks (Pantone, CMYK, Hexachrome CMYKOG) can
be shown in all diagrams.
Best regards --Gernot Hoffmann 
(1)
http://www.google.de/search?hl=de&as_q=&as_epq=CIE+chromaticity+diagram&as_oq=&as_eq=&as_n lo=&as_nhi=&lr=&cr=&as_qdr=all&as_sitesearch=&as_occt=any&safe=images&as_filetype=&as_righ ts=
(2)
http://www.google.de/url?sa=t&rct=j&q=&source=web&cd=1&ved=0CDUQFjAA&url=http%3A%2F%2Fwww. pantone.com%2Fdownloads%2Farticles%2Fpdfs%2Fart_hex_primer.pdf&ei=CTJQUKnOEaWr0QWfg4DoDg&u sg=AFQjCNFx7-5XXealXZGPTQ5ek-A7FWb8gQ&cad=rja
p.2
(3)
http://www.ellerhold.de/index/page/357/subContent/423/index.html
(4)
http://www.fho-emden.de/~hoffmann/labproof15092008.pdf
Edited by author