Knowledgebase
Why do some saturated blues print as purples?
Posted by Larisa Bolli on 27 July 2009 01:28 PM
There are a number of possible culprets with blue/purple issues. The most common is saturated blue colors appearing purple in print, so I'll start there. What happens in this case is that the user defines an RGB blue in Photoshop (or picks a saturated blue Pantone swatch, or some such) and then prints this same color through an ICC profile to a printer, and gets purple. The assumption is that the profile must be wrong.

Actually, what is happening in such cases is that the color selected is so saturated, that it is far outside the gamut (color range) of the printer. So the profile does what ICC profiles do: it uses a Lab colorspace-based conversion to change that out of gamut color to the nearest in gamut color. Unfortunately there is a perceptual issue with the Lab colorspace; the closest in gamut color to many highly saturated blues looks not blue, but purple. You can see this for yourself in Photoshop by having a saturated blue on screen, and using the saturation slider to desaturate it; the result is not a less saturated blue, its a purple. The only difference between the blue in the background and the purple in the forground of the attached image is that the forground color was desaturated in Photoshop (starting with the same color as the background patch), causing the "Lab blue-shift" effect.

You can determine if a given color is in gamut for a given printer/paper/ink combination by using Photoshop's Custom Proof Setup to choose the printer profile, and then the gamut limit tool from the same menu list to see if your spot color turns gamut warning gray (or whatever color the gamut warning is set to in your copy of Photoshop). Or just use the softproof command, and check for a purple shift in the blues. Thats one of the main uses of printer profiles, to allow you to see what your results will look like in advance, so you can adjust them if they are problematic or out of gamut. If a color is out of gamut, then trying to print it is frustrating; the profile must change it, and if its a saturated blue, it may blue-shift in the process. Define an in gamut color of the hue you want, and that should print much more as expected. For photographic colors, use the Select Color Range tool to select the problematic color range, and shift it as necessary.

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