What do the settings in Photoshop's Colour Setup mean and how does it affect the printed page?

All versions of Photoshop have basically the same settings for colour profiles. The difference lies in how you get to those settings to change them. Below are some basic definitions of the particular fields of the colour profile setup. This information is not version specific.

Separation type: GCR

Gray Component Replacement tells Photoshop to replace black with appropriate mixes of cyan, magenta, and yellow. Reducing the amount of black in a photo will help to keep it from becoming too dark. (UCR, or Under Colour Removal is the opposite: Replacing mixes of cyan, magenta and yellow with appropriate levels of black.)

Black Generation: (eg Light)

Black is used in photos as a contrast builder, rather than a major component colour. Selecting "Light" for black generation creates what is called a "skeleton" black plate which is desirable in offset printing.

Black Ink Limit: (eg 70%)

Again, too much black is not good. This setting makes sure that black never, in any part of the photo, goes above 70%. This is an amount that the press can control while still showing appropriate shadow detail. Black inks above 70% in a colour photo tend to destroy detail in the shadow areas, making for a plugged-up, muddy appearance.

Total Ink Limit: (eg 280%)

This setting will ensure that all four inks, when combined, don't equal more than 280%. This setting will go a long way toward compensating for dot gain in most photos, although it will not adequately do so for all photos. Again, each photo is different, and many will require additional adjustments (using Adjust > Curves) in the mid tones.

UCA Amount: (eg 0%)

Under Colour Addition adds cyan, magenta and yellow where black is present in a photo. UCA is generally not desirable in offset printing, and shouldn't be used unless the dark areas are absolutely devoid of those colours. Use UCA only after studying its effects carefully as it can cause disastrous results if you don't know precisely what you are doing. In its defence, however, let it be said that UCA can also turn a flat, lifeless photo into a much richer, more attractive one, if one knows what one is doing.

Ink weight, dot gain, UCR, GCR - what is the difference between newspapers and magazines?

Ink weight refers to the density of ink in a given area of a particular image or file. Because of dot gain and press conditions, newspaper and magazine publishers only allow an ink density of a certain amount. For example, most newspapers are printed on low quality, absorbent newsprint. They generally have an ink weight of about 230%. Magazines are generally printed on coated paper that is of a higher quality and not so absorbent. Consequently, they can have a higher ink density limit of about 300% or even higher.

Once the wet ink is pushed into the paper it can spread. Publishers accommodate this spread of ink by restricting the amount of ink, or ink weight, that they can accept. Should anything exceed that level they run the risk of ruining the print run.

There are specific settings within Photoshop that need to be set to ensure these ink weights are not exceeded in images. See more specific information further on in this document.

In source applications such as Quark, InDesign, FreeHand and Illustrator you must ensure that the CMYK breakdown percentages of the colours you create do not add up to more than the required ink weight.

For example, the ink weight total for a murky brown colour made up of 50% cyan, 100% magenta, 100% yellow and 80% black (K), would be 50 + 100 + 100 + 80 = 330% and is somewhat higher than the required ink weight for newspapers and most magazines. To fix this you would need to reduce the density of each of the plates to reach the required total.

UCR

This is an abbreviation for under-colour removal. It can be defined as a separation technique where black ink is used to replace equal amounts of cyan, magenta, and yellow ink in a neutral to reduce the ink weight. As explained above, publishers have a limited maximum ink weight, above which they cannot print successfully. UCR results in less printed ink, but may sacrifice tonal rendering in shadows.

Dot Gain

Measured by the increase in size of a midtone dot, the spreading of dots during platemaking or on a printing press as wet ink is pushed into the paper and possibly absorbed by it.

GCR

This is an abbreviation for gray component replacement. It can defined as a separation technique where black ink is used to replace either a portion of the unwanted component in a saturated colour, or a combination of cyan, magenta, and yellow equivalent to the unwanted component. Typically specified to improve colour control on older presses.