Re: Gum Color Gamut & Beyond CMYK?
See my page on tricolor gum printing
for a brief consideration of this topic (scroll down the page to the subheading, "Is it possible to reproduce the entire spectrum using three color layers?" )
The answer, from Bruce McEvoy, is no, you do need to add the secondary colors (in other words, in gum printing, use six layers including the secondary colors in addition to the primary colors, in order to produce accurate colors throughout the spectrum, and I suppose some sort of profiles as Jacek suggests might be the way to generate the color separations for the secondary colors. But this seems to me an enormous effort and possibly a misplaced effort to boot; if you're looking for this kind of color precision, gum may not be the right process for you. Not that I would discourage anyone from dedicating themselves to this kind of study if it appeals to them, just that for me and my house, I don't quite see the point.
However, the purpose of the extra colors and separations, as I understand it, is not to extend the gamut so much as to refine it. Depending on the pigments you choose as primaries, the layered color mixtures will produce different palettes of greens, oranges and purples; generally when printing tricolor we pick three primaries that give us the kind of secondary mixtures we want. For example, I generally prefer ultramarine or prussian blue for tricolor because I don't like the greens that pthalo produces with almost any yellow, they look unnatural to me, and since I print a lot of landscapes with trees/leaves, I want a more natural looking palette in the greens. Adding the secondary colors to produce six layers will theoretically make it less necessary to pick and choose primary pigments to produce the kind of secondaries you want, because you can choose the secondaries to your liking. But like I said, this seems a heck of a lot of work, when three colors, chosen carefully, will almost always give us a palette that gives us a reasonable enough approximation of realistic color, with special attention to producing the secondaries that we're most interested in.
On Sep 20, 2008, at 9:11 PM, Jacek Gonsalves wrote: