Maximises the colour gamut of your printer and paper. The colour gamut is basically all the colours that a device can use - so the gamut of a monitor is all the colours it can display, whereas the gamut for a printer is all the colours that it can print (although for a printer, this is also dependant on what paper is used too).

When you create or capture an image, you usually set a colour profile to your image - this is different to the printer profile. It defines what colours can be used within the image i.e. the colour gamut of the image. Typically this is sRGB or Adobe RGB.

sRGB is the most standard, and is best used when you want to show images on the web or to send images to a printing lab (unless the lab specifially states that they will accept Adobe RGB). Web browsers understand what sRGB is, and can display the image correctly.

Adobe RGB is more advanced - it has a larger colour gamut, and so can display more colours. The advantage of this means that with more colours you can get more detail, smoother colour gradients and so forth. The drawback of this is that your image editor must recognise Adobe RGB (and many do now). If you post an Adobe RGB tagged image to the web, you will notice it will be displayed darker than usual - this is because the web browser cannot process and display all the colours. It just treats it as a standard sRGB image, and the colour mapping becomes confused. For your own editing and printing, it makes sense to use Adobe RGB (especially now that many advanced digital cameras allow you to capture images with an sRGB or Adobe RGB colour space - they used to only be sRGB).

So your image now has a colour gamut. Guess what - your monitor has one too! It can only display certain colours. It's not much of a problem though. Monitors can display most if not all of the colours out of both sRGB and Adobe RGB for it to be a non-issue. Some monitors have settings so they can display the whole sRGB gamut, and some even the Adobe RGB gamut. As long as your monitor is properly calibrated so that the colours that it displays are accurate, then you don't need to worry too much about the gamut of the monitor. Do note though that as monitors age, they can wear out. Particularly with CRT monitors, the amount of colours that it can display gets reduced. It might be that the monitor can no longer display the differences in the darker shades. You've then got a reduced gamut, and for accurate editing it would be beneficial to replace your monitor.

The final colour gamut is the gamut of your printer. This can be where the problem is when you're trying to get an exact colour match. Most colours are fine until you get into the very saturated colours. The stronger reds, greens and blues are the colours which cannot always be printed.

As printer technology advances, more and more of these colours can be printed as more inks are introduced. Some printers now have red, green and blue inks as well as the more standard cyan, magenta, yellow and black inks, because they can then print more of the brighter colours - the extra inks extend the gamut of the printer which means more colours can be printed.

The dark colours of the printer can also be a problem. you might notice that your printer with a standard profile does not print all the detail in the shadows of an image compared to what you can see on the monitor.

The colour gamut of your printer is also affected by the paper that you use. If you use very white paper, then brighter colours can be printed as the whiteness of the paper shows through the ink. It the paper is more creamy coloured, then the colours printed will be slightly duller - you certainly will not be able to get a pure white. The paper finish will also have an effect - gloss papers tend to have a larger gamut than matte papers.

So how does the profile help with the colour gamut? Basically a paper profile will try to maximise the gamut of colours that your printer can print. Your printer and paper combinations only have a certain gamut, but your standard printer driver will almost certainly not be using the full gamut - the driver will be on the safe side. With a profile, you should find the gamut gets extended. It might not be by a huge amount, but there will be at the very least a subtle change. There'll probably be more shadow detail, brighter colours, and a better distinction between shades of colours.

Metamerism is most noticeable with black and white prints using pigment inks (such as with the Epson 2000, 2100 and 2200 printers). The vast majority of printers use dye inks which do not suffer this problem. The result of metamerism means that a neutral print might show a green or a magenta tint when viewed under different lighting. The cause of the metamerism is due to some of the colours that are contained in the mix of colours that are used to create the shades of grey. This cannot be solved using a printer profile, but needs a different printer driver program called a RIP. This controls what ink colours are used to make up the grey patterns, and eliminates the colours that cause metamerism. Unfortunately the RIP software can cost as much as the printer.