Why is there colour in my black and white prints?

by: Stephen Crichton on

Have you ever printed out an image in black and white only to see what appears to be shades of cyan and/or magenta?
This can not only ruin a print, but can be very frustration to try to get right.
So why is it happening?
Unless specified, the default setting in most RIP software is to use CMYK in all prints, including B/W. The benefit of this with a B/W image is seen in the light grey areas of a print. When viewed up close these areas are printed using equal values of CMY to give the appearance of grey and allowing for a nice smooth finish.
However if the profile isn't correct, or if the print alignment is out, or if there is too much heat affecting the dot gain of the ink then you may see some colour in an area that is supposed to be neutral.
A work around for this is to try printing using only K.
This will eliminate CMY being used and will give you a neutral print but will create a more grainy appearance in the lighter grey areas as black dots start to separate to give the illusion of grey.
Please see the image below.

Portion A
Using equal values of CMY to give the illusion of grey. Great for a smooth print in light tones but can sometime have issues with maintaining a neutral appearance.

Portion B
Printing with K only. Keeps a consist hue throughout the whole image, but can look a little grainy in light tints.

Your RIP will have a setting called PURE HUE or PURE K or something alone those lines. Select it and print a job for comparison.
Printing with only K should also use a little less ink as it has a lot of white space in the lighter tones.

I hope you have found this helpful.
Feel free to contact me if you have any printing questions.