Continuing the series on RGB and CMYK colour systems, with useful tips for creating print artwork for CD & DVD replication, packaging or other formats.
One of the most common points of confusion when using CMYK is the best way to produce Black. Unlike RGB there are actually two general methods:
1) Remove all Cyan, Magenta and Yellow, then increase Black to 100%
2) Use some combination of all colours, with dominant Black e.g. C 60% M 60% Y 60% K 90%
To complicate matters both methods are perfectly acceptable but each are ideally suited to particular situations. Below I'll explain when these should be used and why.
Method 1 - single colour 100% black (C0 M0 Y0 K100)
This method is ideal for black text or small detailed objects, or for barcodes where you need objects with sharp lines. For an example of this method look closely at a newspaper or magazine and note how the text in the main articles is solid and sharp while most of the coloured/white text in adverts is made of dots and is slightly fuzzy.
When reproducing colours in the CMYK system different tones are created by overlaying halftone dot patterns that are small enough and numerous enough that we perceive a consistent image. For example, C50 M50 Y0 K0 will look like a purple tone. The problem for detailed objects is that dots will not provide a strong outline - you get shapes which look slightly fuzzy at the edges. By using a single colour at 100% the object will be reproduced as a solid area, rather than one constructed using dots, which allows a strong clear outline.
Looking at printer technology we see another potential issue with dot printing colour mixes. Each colour (C,M,Y,K) is applied from a different plate at different times. In some less rigorous printing processes you can get positional mismatches between the different colours which is another source of potential blurriness around the edges of detailed objects. This is particularly common in local newspapers, but in our line of work we use more advanced systems. Nevertheless, it's good practice to think about this issue and produce text in single colour format, rather than a dot mix.
Method 2 - Deep black from mix e.g. (C60 M60 Y60 K90)
Above we've looked at some problems with using a colour mix to produce black, particularly small black text, but there are situations where it is the preferred method. The problem with the single colour black is that, in practice it produces a dark grey rather than deep black. Essentially the layers of ink applied to a replicated CD or DVD, or printed packaging item are quite thin so you will get some of the (white) surface shining through the black. This produces a visual mix between black and a small amount of white which, as we all know, will look like a grey.
The way around this is to add in the other colours on top so less light shines through, giving a deeper looking black. This setup is ideal for large areas of black which you want to have a bold appearance, though as noted above not ideally suited to small detailed objects. In smaller objects the 'greying' effect is not noticeable anyway.
There are a number of combinations you can use to give a similar effect - the one suggested above, C40 M40 Y40 K100, Photoshop's default black C75 M68 Y67 K90. Or you could ask your printer what they would recommend for a deep black.
