Color spaces

A PDF document can contain objects of different color spaces: Separation, DeviceN, CMYK, Gray, calibrated Gray or RGB, Lab or ICC-based colors. The color space of the object has an impact on the object’s overprint behavior. The following rules apply here:

Color space	Object type	Overprint mode	Rule
Separation DeviceN	Irrelevant	Irrelevant	Only those inks are specified as listed in the respective color space. Separation Black, for example, will only have the black ink specified, not cyan, magenta or yellow.
Gray RGB Lab ICC-based colors Calibrated Gray or RGB	Irrelevant	Irrelevant	Cyan, magenta, yellow and black inks are specified. To determine the percentage of the inks in non-CMYK objects, the colors in these objects are converted to CMYK according to the normal color conversion rules. An object in Gray, for example, may have the following inks specified: C: 0 % M: 0 % Y: 0 % K: 60 % Consequently, objects in Gray will always knock out the underlying cyan, magenta or yellow, regardless of the object type or overprint mode.
CMYK	Image Shading	Irrelevant
	Text Line art Image masks	Standard (OPM 0)
	Text Line art Image masks	Illustrator (OPM 1)	Only those inks are specified of which the tint value is not equal to zero.

We learn the following from this overview:

Only CMYK text, line art and image masks have an overprint behavior which depends on the overprint mode (OPM 0 or OPM 1).

There is black and there is black. Objects in a spot color named “Black”, in the Gray color space, or in the “Black” ink that is used to render the K plate of CMYK have the same overprint behavior, and the overprint mode (OPM 0 or OPM 1) makes a difference for CMYK Black. Objects in Separation Black or in Gray, however, will overprint differently.

Ink	Object in spot color black, Gray or 100 % K	Object in Separation Black
Cyan	0 %
Magenta	0 %
Yellow	0 %
Black	100 %	100 %

Be careful with color conversions, because the overprint behavior may be unpredictable. If you do color conversions, proofing is an absolute necessity. You can proof your PDF documents by printing them on a composite output device, or on screen using the Overprint Preview and Separation Preview features of Adobe Acrobat.