CMYK, cyan, magenta, yellow, black. Is the four-color process system. Any photo-realistic color gets simulated by combining tiny dots of these four inks. CMYK is flexible and economical but has limits: it can't hit every PMS color exactly, it can't be metallic or fluorescent, and it's not opaque on clear or dark substrates.
A spot color is a single ink, pre-mixed to a specific formula, loaded onto its own plate, printed at full strength. It hits the exact shade every time. It can carry metallic pigment, fluorescent pigment, or opaque white base. What it can't do is blend with other spots to simulate photography, that's what CMYK is for.
Most commercial print jobs use both: CMYK for the imagery, plus one or two spot colors for the brand logo, the metallic foil, or the opaque white layer under the artwork.
In PDF terms, a spot color is a Separation color space. A color defined by name plus an alternate CMYK or Lab recipe used for on-screen preview. The press only uses the name; it knows "PANTONE 485 C" means "load the PMS 485 C ink pot."
When a preflight tool reads your file, it lists each Separation color it finds. That list becomes your plate count. If you have CMYK plus two named spots, the press will set up 6 plates (4 process + 2 spot). Specialty spots, "White," "Foil," "Varnish", each count as an additional plate.
Swatch left in the file that isn't used. A Pantone swatch defined in the Illustrator swatch panel but never actually applied still shows up in the PDF's separation list. The press gets confused about whether to set up the plate. Clean the swatch panel before exporting.
Wrong Pantone suffix. "PANTONE 485 C" is the coated-paper formulation; "PANTONE 485 U" is uncoated; "PANTONE 485 M" is matte. They're different ink formulas. Using the wrong suffix for your substrate means the final color won't match.
Duplicate spot colors. Two swatches named "PANTONE 485 C" and "PMS 485" are the same ink with different names, but the preflight tool sees them as two separate plates. Standardize naming across the whole file.
Mixing spot and process for the same element. A logo built partly in spot and partly in CMYK won't render cleanly. If the logo is spot, keep it 100% spot.
Preflight identifies every spot color in your file, named Pantone Colors, CMYK-derived spots, specialty inks, and white underbase, and gives you the full plate count before you send it to press.
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