Assigning a profile does not change the original data. It does change the interpretation of that data, though, so it looks different.
Converting to a profile does change original data, so that's potentially destructive and shouldn't be done unnecessarily.
Assigning color profiles is a permanent/destructive action? I mean, is it possible to change a color profile with no permanent changes in the original file?
thanks for the response, does it mean that I can 'label' a file with different color profiles for different output devices? I have only one photo and I have to change it for different outputs and I don't want to make too many digital copies for each color assigning
No, it does not. You don't assign for output, you convert.
Keep one master file in a color space large enough to contain all the data, and then you save out copies converted for specific output. No need to keep these copies, they can easily be recreated as needed.
Bruce Fraser summed this up in an Adobe forum:
"You could change profiles a thousand times via Assign Profile and the image would not degrade... inasmuch as the numbers in the file wouldn't change, this is true. But it would display incorrectly, and convert to any other space incorrectly, so it's fair to say that while the integrity of the data hasn't been compromised, and you can rescue the file by assigning the correct profile, for all practical purposes, it's hosed."
IF all images were properly tagged with an embedded profile, there would be zero need for the Assign Profile command. Where it's useful and necessary is the rare case that a document has no embedded (tagged) profile; RGB mystery meat as Bruce liked to say. The RGB (or CMYK) values have no scale. It's like asking me how far I live from you and I simply write: 1000. 1000 feet, miles, kilometers? 1000 without a scale has no meaning. As does R34/G99/B200 without an assigned profile to define it's scale, it's color space. Now assign sRGB to the numbers, or Adobe RGB (1998) etc, we have a scale (miles, kilometers etc).
do I also have to save a master file for each color assigning? that's a bit confussing for me
No, what I'm saying is the opposite: Keep one master file. Say you have that in ProPhoto because you have some very saturated colors there.
Then you need to post it on the web - so you save out a copy, converted to sRGB. The conversion may introduce channel clipping because sRGB is much smaller than ProPhoto, but that's a different and separate subject.
Next, you need to send it off for offset printing in a book. So you save out another copy from the master, convert it to Web Coated SWOP, ISO Coated, FOGRA39 or whatever the printer wants.
There's normally no need to keep any of these copies. They can be recreated from the master file at any time. Just delete them when done.
And just so we're absolutely clear on this: It's always convert, not assign - except in one special case:.
If the material is CMYK and you have elements that are to be overprinted on the black plate only, like text or other graphic elements, you need to assign and readjust colors if necessary. That is, if it turns out that you have the wrong CMYK profile.
In this case a normal conversion will turn K-only black into 4-color black, which you don't want.
is it possible to change a color profile with no permanent changes in the original file?
for additional clarity:
Photoshop> Edit> ASSIGN Profile (is the move my Bruce Fraser's text was referring to)
Photoshop> Edit> CONVERT to Profile does change/degrade the color (if the conversion move is saved, the damage is permanent and cannot be recovered. if the conversion move is not saved, the degradation still occurred in the open document, but is not permanent because it was not saved)
generally use and maintain the original embedded profile (color space) -- convert to a destination profile only as needed -- keep profile conversions to an absolute minimum because the clip or compress color information
RENDERING INTENT is the term for how Photoshop (and print utilities) allow us to control the color re-mapping/transformation process during a profile conversion
two common rendering intent options are PERCEPTUAL & RELATIVE COLORIMETRIC
perceptual is said to "compress" the colors into a smaller color space
relative colorimetric "clips" the information and discards it
if you are unsure which rendering intent to use, try each one and compare the results