Tutorials

A bit of information about Colour Management.

Colour Management

What is Gamut?
Gamut is the range of colours that can be represented by an input or output device.

What is a Colour Profile?
The gamut of colours used must be mathematically transferred from one device to another in order that the digital information can be correctly interpreted into colour. That is done by embedding a colour profile into the image file (metadata). These profiles are called working spaces and sRGB, aRGB, ProPhoto, Wide Gamut RGB are just a few. There are lots and lots!
If no colour profile is embedded most software will assume sRGB and Microsoft (XP) was almost entirely colour blind.

How are the colour profiles interpreted?
The software you use has a routine called a Colour Engine
The colour engine mathematically converts from one colour space to another.
Photoshop allows you to choose which colour engine to use, if you have more than one. They do not necessarily produce the same end results and they can be instructed to convert colours from one space to another in different ways. Photoshop offers 4 methods (Intent) for converting plus a few other options.
Some understanding of colour management is required to delve in here, don’t ask me.

Does your brain hurt?
It need not, just stick to sRGB throughout your workflow then once you are set up correctly you will have no problems.
If you are using something other than sRGB and the above is grandma and sucking eggs please read no further (unless you want to help correct errors) and carry on using your colour space.
If you are not using sRGB and your brain hurts, then think carefully about your path forward. In the history of the club most complaints about poor colour reproduction have come from members using aRGB and not fully understanding the consequences. The club’s PDI software does now convert from aRGB to sRGB for projection. Not all clubs use this software so it may look fine at the club and wrong at another club. Also the ‘Intent’ of that conversion isn’t known at the time of writing this.

Why use anything other than sRGB?
This is a bit of an emotive area.
You are not a real photographer if you use sRGB.
You are throwing away half your colours.
Why buy an expensive camera and not use all of its capability.
The web is full of very vocal blogs and articles that boil down to one of the above and some of these folk are not rational in their beliefs. It is a bit like talking to an Audiophile on the finer points of analogue vs CD music or asking a Canon (or Nikon) owner for advice about which camera to buy. There is a lot of myth and magic out there!

What is Gamut Warning? (cmd shift Y on a Mac)
In Photoshop there is a facility for checking if your printer (provided correctly installed and colour managed, .icc profile for printer and paper installed) will reproduce the colours in your image correctly. If you don’t use sRGB and don’t know about Gamut Warning, your brain should now start to hurt.
The club had a colour expert come and give a day seminar on colour management (twice). That was where I discovered Gamut Warning as in his workflow he checked Gamut Warning and he then desaturated the colours highlighted (in grey). This meant that he was confident that he knew what the colour gradients would be like. If you do not use Gamut Warning then the software you use or the printer will adjust the colours in order to cope with out of gamut colours. That isn’t necessarily under your control. Gamut warning is therefore a tool to investigate colour handling. There is also soft proof (Proof Colours) which allows you to see on your monitor what your print will look like when printed. This uses the colour profile for the paper and printer you use (sometimes called an .icc file) Soft Proofing reproduces on the monitor the effects of your inks on your paper, clever that.

What can my equipment reproduce?
This isn’t actually that easy to determine.
I have an expensive monitor, it is sRGB, not aRGB. So if I try to use aRGB I am not getting WYSIWYG and therefore I have to try to imagine what my output will look like. Since I purchased my monitor new versions have arrived which partially cover aRGB however they cost over £1000!
My printer is an Epson 3800 and this is (apparently) capable of covering more than the sRGB gamut but less than the aRGB gamut. I have looked up the 3880 which is its replacement but Epson decline to clearly state the gamut it can achieve. So to be confident it looks like I need to stick to sRGB.
My camera allows me to process its files to produce images with the embedded profiles sRGB, aRGB and ProPhoto. However I cannot find any information as to how well my camera output can reproduce the colours in those colour spaces. I do know that no camera can completely cover ProPhoto as that includes ‘Imaginary Colours’!
I looked up DS colour labs and they advise a workflow using sRGB which I guess means that their expensive Fuji printers only cover the sRGB gamut.

If I use sRGB does it matter?
I cannot answer that question for you. I do know that I don’t need aRGB, or any other colour space that is larger than sRGB. I know that because I have looked at the kind of images that I like and processed them in more than one colour space and on the whole the colour gamut required by my images is covered by sRGB (using cmd shift Y). I had to use test images (sRGB, aRGB, Wide Gamut RGB) to test the capability of my colour printer and what was scary was the amount of the sRGB test image that went grey when checking Gamut Warning.
The advantage of sRGB is that you do not need to have a comprehensive understanding of the colour train to obtain reproducible results and your camera, monitor and your printer are probably (only) sRGB capable.

The human eye is pretty easy to fool when it comes to colour, it is not an absolute instrument, it can compare quite well. So if the image you print looks OK to others then that is probably acceptable. If you were photographing for a catalogue showing women’s clothing then a full understanding of the colour train and the printing process would be essential!!! I’m still not convinced you would need to go beyond sRGB.

I wrote this tutorial after seeing how many entries were being put into PDI competitions using aRGB. I would like to state that I am not an expert on the colour train and I am more than happy to be put right by anyone that KNOWS. It is easy to do experiments at home and find out if you need anything more than sRGB.

Since I originally wrote this I have attended a colour management course and done a few more experiments and learnt a bit more.
The gamut check can mislead and needs to be used with a bit of care as the amount by which a colour is out of gamut isn't indicated. Although in a graduated area the grey does become more intense as the gradient area gets more out of gamut.
I took the test image from Native Digital which is in aRGB. I processed it in a number of ways including converting it to sRGB. I cannot see any significant difference in the printed aRGB and sRGB versions where the work flow is the same other than the original colourspace. As this image is pretty garish I challenged the instructor of the course (by email afterwards) to provide an aRGB image which when printed looked better than its sRGB version. He hasn't sent one but sent the following comment, "I can appreciate that the difference between Adobe RGB and sRGB is very subtle, often invisible." So if you are an aRGB advocate and believe you have such an image and are prepared to part with it please share so I can try printing it out.

I would also say that if you have a workflow process which uses other colour spaces and it works for you then stick with that workflow.

Here are a couple of articles which you might like to read Cambridge in Colour and Ken Rockwell. Enjoy.

Tony Kaye is in the RPS Scientific Imaging Group, is well informed and has a thread on sRGB vs aRGB HERE

Rex