The preceding paragraph may seem a little counter-intuitive because a converted image will look the same on the Photoshop screen before and after a conversion. If, however, you were displaying an sRGB image and assigned a different profile without also converting, then the image would look different before and after the assignment, even though the image file data did not change in any way. Assigning does not change the image data, but converting does change it.

As a further illustration, let's look at the dialog box from Photoshop when you try to open an image that is already tagged as an sRGB image, but the working space in Photoshop has been chosen to be Adobe RGB by the user.

Profile Mismatch Dialogue

In this case the user wants to open a camera image already tagged as sRGB, but the user has also told Photoshop that Adobe RGB is the preferred working space. Photoshop is quite happy to do any of the following:

  1. Open it as an sRGB image anyway, because Photoshop can open multiple images all in their own tagged working spaces.
  2. Convert the image to Adobe RGB, tag it as Adobe RGB, and then open it.
  3. Just open it without any worry as to how it will look on the screen. Again, this is most useful for web images where exact colours don’t matter. (Very few web users have calibrated monitors anyway.)

Monitors and Printers

A colour-managed application can also deal with ICC profiles for output devices such as monitors and printers. Without colour management, no two monitors will display identical colours. When you email Grandma a JPEG photo, you have no idea what she is seeing on her screen. Ever been in an electronics store and watched the same football game on twenty different TV sets?

Printers are the same. Without ICC profiles and colour-managed applications, what you see is, likely, not what you get. In a fully colour-managed environment you can take a photo, view it on your screen, and print it out with all the colours matching at every step of the process except in situations where the colour gamut of, say, the printer and paper combination doesn’t quite match the gamut of the working space. In these cases, colours that cannot be exactly matched will be modified to fit into the printer space. Using the gamut warning in Photoshop proofing will tell you where colours cannot be matched.