Certain jobs demand Spot color. Modern proofers ( printers ) use a RIP to process the Spot Colors and use internal color tables for conversion to output. So, you used to have to create CMYK swatches generated by the printer manufacturers to match Spot color more closely, but that has all changed with modern proofers. I'm trying to explain this quickly because of time, but lets keep the conversation going. I know exactly what you mean and why you are confused. A lot of this has to do with how the file will ultimately be printed. For instance, a stationery job will consist of a letterhead, an envelope, and a business card. You want to keep color consistent in all three pieces and hopefully using the same type of paper throughout. Most offset printers will print you job using Spot colors, probably 2 or 3. Now, you've got a crappy inkjet and need to show the client a proof ( hypothetically speaking ofcourse ). If you create your file using Spot color and send it through that inkjet, you will get unpredictable results ( depending on the quality level of the printer, its driver and or its RIP ). Pantone gives you CMYK conversion percentages that you should use on such a printer if it doesn't have a Postscript Level 3 RIP. If it does, then you can keep the Spot color in the file and the RIP will do the conversion for you.
Thanks John. It is interesting that you brought up Pantone, as that is not what I had in mind. So perhaps there is actually a THIRD definition of "spot":
1) the old school definition: things you use when you want a a separate plate and a special ink.
2) the one I don't understand and asked about in my original question.
3) a synonym for colors chosen from a Pantone book.
It would be most helpful for me if we could leave Pantone out of the discussion.
Imagine I have a set of fabric swatches. I scanned them with a spectrophotmeter and converted the spectal data into LAB D50 and confirmed that the colors are all within the CMYK gamut and so will print correctly if I can express them correctly.
I now want to create a set of swatches I can use in PS, ID and Illy. In another forum and was told that I should define those swatches as spots if I want to keep the the LAB intact.
I don't disagree with that advice...but it did make me realize that my understanding of the definiton of "spot color" is antiquated.
Why not just define those swatches as L*a*b and add them to the swatches or create a new swatch set. They'll be automatically converted to the proper RGB or CMYK according to the profiles loaded in your Color Settings. If they already are in CMYK gamut, is there any real point in defining them as spot.
That is what I thought, too. So I made a set of colors defined as LAB but set as process, not spot. Saved them in their own swatch file. Sychronized the color management settings in ID, Illy and PS using Bridge. Opened a new document in each and loaded the swatch file and checked the LAB numbers. They were no longer the same. I posted a question about this in another thread and was told the LAB definition stays intact in any color mode and color space if the swatch is defined as a spot color only. Hence my confusion.