[cairo] Spot colors (and CMYK)

[Chris Murphy]

On Feb 24, 2010, at 2:22 AM, Jon Cruz wrote:
> 
> I don't think you've been getting the print world application I'm trying to be able to support.
> 
> The main point is that in *practice* graphic designers do not overlap CMYK with spot color. They trap them to avoid such issues.
> 
> So in the print world the situation you seem so worried about is one that is normally avoided. Just as is 100%C, 100%Y, 100%M 100%K is avoided as in the real world that will usually oversaturate the printing medium with ink.
> 
> Oh, and again keep in mind that normal offset printing is incapable of reproducing many spot colors. So often "mixing normal colors" fails to reproduce the targeted value.

Yeah. The most common use for spot colors is for logos, and solid illustrated (vector) areas. Process builds (CMYK) tend to meander as a press is running, and depending on the color, this meandering may be unacceptable so a spot color, which doesn't meander, will be used. This is why you will find even marginally reproducible pastels printed as spot instead of CMYK. A light beige can appear slightly green (and hence ugly) just with a small drop in magenta ink film density on press, and then it can tend slightly pinkish with a small increase in magenta ink film density.

Interactions of "spot colors" with each other, and CMYK is probably most commonly implemented by CMYKGO which is called Hexachrome. But in this implementation, green and orange are not really considered spot colors, they're considered additional primaries. So it's a 6 primary printing process, instead of 4. You can't really arbitrarily choose additional primaries or you'll just make the job more expensive for little gain, so outside of Hexachrome, it's somewhat rare to cause spots to interact. Maybe the next most likely case of intentional interaction is where you have a spot metallic silver color, and then you're going to print one or more colors on top of it so you can create the appearance of a metallic pink, or yellow, or green, etc. 


Chris Murphy
Color Remedies (TM)
New York, NY
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Co-author "Real World Color Management, 2nd Ed"