This picture shows again a repesentation of the gamut of the sRGB colour space showing the most saturated reds, greens and blues at the pointed parts of the picture.

Roll your mouse over the picture to see how much more saturated are the greens contained inside the larger AdobeRGB colour space (wire frame).

Click to see how much more saturated reds, greens and blues are contained inside the much larger ProPhotoRGB colour space.

The picture above also shows clearly how the same colour value (0,255,0) represents different colours in the sRGB, Adobe RGB and ProPhoto RGB colour spaces.

When you use colour numbers it is really important to know what space the colours are encoded in. By way of an analogy, 100 kph is a very different speed from 100 mph. You'd better know what measure has been used to calibrate your speedometer before arguing with the traffic police!

The colours on this picture demonstrate the idea rather than the actual colours represented by 0,255,0 in sRGB, AdobeRGB, and ProPhotoRGB colour spaces. Also, for the sake of the example, I've compared the colour ranges using an 8-bit space. In reality, you would never try to represent the ProPhoto RGB space with only 8-bit numbers because there would b too few numbers for so many colours. The colour 'distance' between adjacent numbers would be very large, and colour transitions would not be smooth. The ProPhoto RGB colour space is always referenced to 16-bit numbers in practice.

However the important point is that the same numbers represent different colours in each colour space. That is why it is so important to specify which colour space is to be used when interpreting colour numbers.

If you send an image file encoded in AdobeRGB or ProPhotoRGB to someone to print, and they interpret the numbers as sRGB numbers, every value will point to a less saturated colour than you intended. Your image will be printed with dull flat colours.

Roll your mouse over the image of the flower to see what happens when an AdobeRGB file is mis-interpreted as sRGB.

Click on it to see what happens when a ProPhotoRB file is mis-interpreted as sRGB.

On the contrary, if you sent an image file encoded in sRGB to someone and they interpreted the numbers as ProPhoto numbers, your image would appear grossly over saturated.

flowersrgb

Properly colour managed processes have two protections against this ghastly error.

A computer application that is "ICC aware" — Photoshop, Lightroom, or Aperture for example — will always attach (or embed) a 'tag' in an image file to say how the file should be interpreted. "ICC aware" applications always look at the tag when opening an image file to see how its colours should be interpreted and, unless instructed by the user to "ignore embedded profiles" will interpret the colours correctly.

Beware of operators who tell you they "always ignore that profile stuff". You will be disappointed.

Colour-conscious photographers will also be wary of advice to "do everything in sRGB to avoid problems". Followers of that advice trade off the ability to produce the widest colour gamuts in their print photography.

There are good discussions of the benefits of using the widest possible colour gamut in your workflow, in document and on site.

Computer applications sometimes also give you the option to 'assign a profile' to a photo file, or to 'convert' a photo file 'to such-and-such a profile'. The frequently-asked question, "what is the difference beween 'assigning' a profile, and 'converting to' a profile?", is answered here.

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