I should expand upon my earlier comments a little bit for clarification. For those not familiar with profiling or moderately advanced color theory.
Most all inks experience some form of metamarism. Normally the color shift is too slight to really notice. A minor shift in hue in a saturated red is still a saturated red after all. The areas where metamarism are most visibly apparent are when you make up grays using the CMY inks. It is a delicate balance to take three primaries and turn them into a neutral shade of gray. Any variance will impact the neutrality and show up as a color "cast". Either greenish, warmish (red), or cool (blue).
Normally the profiles we use in our inkjets use "Gray Component Replacement" (GCR) to some degree. GCR is where you use black ink to make up the grays in your image. But why wouldn't you use black to make up all the grays in the image you might be asking yourself? The reason(s) are that you simply don't get the smoothness in tone and transition, nor the same density and "pop" as you do with the preferred method of using CMY to create the neutral tones. Plus there's the always present dots that we have for years been trying to eliminate visually in the quest to mimic the continuous tone of a true photographic process. CYM dots have less contrast against a white background than a black dot does. Therefor they appear smoother on the sheet.
The use of light cyan and light magenta are simply a way of reducing the contrast of the dots against the sheet even further. We don't use a light yellow because there's so little to be gained from the additional channel in smoothing the image as yellow has such little contrast against white to begin with. As can be seen in the world of aqueous inkjets the addition of a light black, and in some cases light, light blacks (and beyond) allow us to use more GCR lower down the range without increasing the visibility of the dots. Mimaki, I believe, is the first to implement this with their JV5 series printers and an available solvent light black inkset.
Of course this whole visible dot issue seems pointless for a sign printer where most prints are viewed from distances that obscure the dots anyhow. But if you were say producing small decals where the customer is viewing them close up it does become an issue. Smoother dots appear more like a true spot color. The other more useful benefits of GCR, where they intersect with with the signage industry. Are that it makes neutrals easier to print consistently with a device that isn't custom profiled, as the CMY balance become less critical. Or the topic of this thread where it's used to minimize the effects of metamarism.
Even with the addition of light black inks there will still be the need to use CMY as part of the gray component. The increased density across the tonal range is visually superior to the alternative of using a single ink to produce grays. Compare a completely neutral black and white image printed using black only to one that has been generated from an RGB file and printed using CMYK and you can easily see how much more contrast is apparent in the image. It simply "pops" by comparison. However, the addition of a true neutral component to this mix allows the reduction of the CMY portion and minimizes the metamarism in a correctly profiled and printed image, as well as making a less accurately profiled machine able to print a neutral tone with less color casts should the CMY balance be slightly off. The trade off at the moment is an increase in graininess due to the use of the black dots lower down the tonal range. Something that may not even be an issue for some here. For others it may create certain solids that appear too grainy to be salable.
The answer for those of us who stress over accurate color and create our own profiles, is as easy as using the same target data to create two different profiles. One using less GCR for a smoother image, and another using more GCR for better neutrality under different lighting conditions. It adds less than 5 minutes to the profiling process and gives you the flexibility to create a range of products that will keep everybody happy. Although the true answer lies in the technology of expanded inksets that include lighter neutral inks. Something to consider when making your next printer purchase, if any of this lengthy diatribe has struck a chord with you.
