ICC color profile management
What is the importance of ICC color profile management anyway? In simple terms it is alike hi-fi stereo, where high fidelity is maximized from source to speakers, passing through microphone, recoding media, player and amplifier. Much the same, ICC color profile management tries to maintain color fidelity from photographic subject through the camera to the output media screen, print, paper and beamer. Luckily digital photography has taken out one source of distortion, the storage media (as the CD-ROM did in the audio field).
The figure below images the problem: Every time there is a transfer from one device to another, an ICC profile is used to compensate for the inaccuracies or limitations of the device. The central work space (which is called Profile Connection Space, PCS) provides a common platform to translate the device color spaces into each other.
The inter-connection between devices and their color spaces
CM Pros. CM is really important whenever you need identical results between shooting sessions, accurate source reproduction, close rendering results between various display media (this would rather point CM to professionals).
As a passionate amateur you can also profit from CM, since it is mostly associated with 16 bit color depth. RAW mode shooting is much more forgiving in terms of exposure than processed shooting (JPEG), so you can generally underexpose and recover the highlights. And, the human eye is much more sensitive to luminosity variations in the dark scales than a digital camera. Careful tonal adjustment close to the blacks will produce a better dynamic of the image. That is why RAW mode images often have an appearance of more depth to them.
CM Cons. If you do not use color management you still can realize fantastic images. In journalism or emotional contexts, for holiday photos you do not need any color management.
To configure digiKam for ICC color management, please refer to the setup section.
The ICC standard covers a data format to exchange color information of devices. ISO 22028-1 specifies unambiguous exchange of color image data of color space encoding, viewing conditions, image state and reference medium. Here follows an example of the differences in color language: one kind of green defined (by the same numbers) in one color space looks different in another color space. This is what happens when no color management is applied.
(88, 249, 16) in Adobe RGB 
The same RGB value in sRGB. Here is a link where you can play with color spaces.
Photographers want to use the full gamut of their camera and their ink jet printers. Editing of images should be done in a work space, where equal RGB-numbers result into gray (like #333 or #CCC).
The following diagram tries to outline the logic digiKam will follow in its CM work flow, depending on the settings made in IO Files and ICC profiles of the configuration page.
Soft Proofing is a way of previewing on the screen (monitor) the result to be expected from an output on another device, typically a printer. Soft proofing will show you the difference to be expected before you actually do it (and waste your costly ink). So you can improve your settings without wasting time and money.
Rendering intent refers to the way gamuts are handled when the intended target color space cannot handle the full gamut.
Perceptual, also called Image or Maintain Full Gamut. This is generally recommended for photographic images. The color gamut is expanded or compressed when moving between color spaces to maintain consistent overall appearance. Low saturation colors are changed very little. More saturated colors within the gamuts of both spaces may be altered to differentiate them from saturated colors outside the smaller gamut space. Perceptual rendering applies the same gamut compression to all images, even when the image contains no significant out-of-gamut colors.
Relative Colorimetric, also called Proof or Preserve Identical Color and White Point. Reproduces in-gamut colors exactly and clips out-of-gamut colors to the nearest reproducible hue.
Absolute Colorimetric, also called Match or Preserve Identical Colors. Reproduces in-gamut colors exactly and clips out-of-gamut colors to the nearest reproducible hue, sacrificing saturation and possibly lightness. On tinted papers, whites may be darkened to keep the hue identical to the original. For example, cyan may be added to the white of a cream-colored paper, effectively darkening the image. Rarely of interest to photographers.
Saturation, also called Graphic or Preserve Saturation. Maps the saturated primary colors in the source to saturated primary colors in the destination, neglecting differences in hue, saturation, or lightness. For block graphics; rarely of interest to photographers.
