Is accurate colour reproduction possible?
The answer must be no for many reasons, principally because the perception of colour is subjective to the viewer and no two people could agree on this or that colour actually being this or that colour.
One thing is certain, the pre-requisite for assessing colour is the ability to see colour. It's amazing how many people are colour-blind to some colours, the difference between Blue and Green being a common example. Without meaning to be offensive I would reccommend everyone, wishing to study this topic, to get their eyes tested for colour-blindness.
The classic test is the Japanese 'Ishihara' colour charts and if you think I'm writing a load of rubbish go to http://www.toledo-bend.com/colorblind/ishihara and find out. Don't forget to comeback.
Pat yourself on the back if you pass all the tests but don't worry if you fail some of them as there are ways to compensate. The importance is knowing one's limitations.
This tutorial is not just for Photoshop users nor people that work solely on computer screens. The principals involved in accurate colour reproduction apply to any form of photographic image be it on screen or on paper, both photographic (light sensitive) or printable by pigmented inks such as Inkjet Printers or, in offset photo-lithographic reproduction such as magazines and even your T.V. screen. Though Photoshop, by it's popularity, has become the 'de facto' facility for image colour management, it's of no use whatsoever if you don't know where you're going because you don't know how to get there.
So the purpose of this tutorial is to instill an understanding of photographic colour to the point where one can recognize the faults in an image and immeadiatly apply the right amount of the right correction.
Inextricably linked to Colour is Brightness and Contrast so let's deal with these first because, as you will see, incorrect brightness or contrast could give you a false impression about colour accuracy. The image is either:-
Too Dark Too Light Just Right Too Contrasty Too Flat
No shadow detail No highlight detail Dark shadows Detail everywhere
Dark Highlights Detail in shadows Burnt out highlights No Sparkle
The Tricolour Chart shown again left is a simplified concept of the behaviour of light using just three colours Red, Blue and Green, the Primary colours in Photography. A mixture of various wavelengths of light, each representing a colour, combine to create White Light in the central patch. Where only two colours combine there remains the complimentary colour of the one that's missing. In other words, where only Green and Blue coincide it creates the colour Cyan, the complimentary colour of Red, being absent.
Red and Blue create Magenta the opposite of Green. Red and Green create Yellow the opposite of Blue. This RGB system forms the basis of Adobe's RGB/sRGB standardized colour pallettes.
The complete opposite of this is the Complimentary colour, CMY chart, shown right, where all colours combine to create Black, a colour desinated 'K' hence the familiar term CMYK.
This system is of more use to Offset Litho printing with pigmented inks, I include it here merely to further illustrate the concept of colour combinations. These six colours and their combinations are the only ones you need to deal with and if an image has any kind of colour 'cast' there are only twelve possible variations.
Notice how the Contrasty image has assumed a 'warm' look. The Ladies lips are Redder, her skin tone Yellower. To a lesser extent it also appears in the Dark image. This is merely an effect of incorrect brightness and/or contrast. From this one can understand the importance of a having an image with the correct tonal quality before applying critical colour correction.
CYAN
YELLOW
MAGENTA
RED RED/MAGENTA RED/YELLOW
BLUE BLUE/CYAN BLUE/MAGENTA
GREEN GREEN/CYAN GREEN/YELLOW
I admit it's not always so easy to see a colour cast especially when the subject matter is unrecognizable however, notice how, in the green image, the Emerald is quite pronounced as you would expect, but the Saphire is desaturated. Then in the green/cyan image the Saphire is more saturated because of the blue content in cyan. Cyan being the opposite of red has left the Ruby even more desaturated than the green image.
Again, in the red/magenta image the Ruby looks vibrant but the Emerald has almost completely lost it. Look at the Emerald in the red image then in the red/magenta image, there's a big difference and notice how the yellow Gold seems to have dissappeared from all the blue cast images. Practising like this is the only way to learn.
A word here about terminology. For obvious reasons one cannot have say, a blue/red image because that would be magenta or a green/magenta image, being opposites but one can reverse the order. Instead of blue/cyan it could be cyan/blue, depending on the predominance of colour cast, so on and so forth.
This all depends upon which industry you are in and how one alters the colour, for example, in the photo-finishing industry the printing machinery has only cyan, magenta and yellow controls and one alters colour by subtracting or adding perceived colour in CMY mode. This means that if the image is blue/cyan then you should reduce or subtract the blue/cyan content but, think logically now, one can achieve the exact same result by adding yellow/red! Or a green/yellow image can be corrected by adding magenta/blue. Different industries have different methods but the important point here is that you only need three controls either Primary RGB or Complimentary CMY. It's good discipline to decide which you want to work with and stick to it, apart from reducing the confusion factor if dealing with six colours.
In Photoshop you have a choice. As shown in this not very good quality screen grab of the Colour Balance dialogue box you can correct a blue/cyan image subtractively by reducing cyan (adding red) and reducing blue (adding yellow) or additively by doing exactly the same thing!
The next thing to know is important. One must only alter two colours at the most. One cannot have an image that is blue/cyan/green or red/magenta/yellow, for examples, as these are contradictions. (* See footnote about mixed lighting) Try to understand this, if you increase red, green and blue by equal amounts you will not alter the colour balance, you will just add density to the image i.e. make it darker. Conversely, increasing cyan magenta and yellow by equal amounts is the opprsite and will merely lighten the image. Remember you are dealing with Light not Paint. Altering three colours by varying amounts will only introduce an amount of darkening or lightening to the image but will leave a colour cast. Try this in the Levels dialogue box in Photoshop and see.
Now we come to the part that is hardest to grasp.
What colour is this image?
Well done if you think it's red/yellow
Is it red/yellow in equal amounts?
Is it red/yellow predominantly red?
Is it red/yellow predominantly yellow?
(In which case we would say it's yellow /red)
And the big question - by how much of each?
Looking at the picture we see a Baby Girl with a hat that could be blue. She has blonde hair and light skin. The predominant feature is obviously the face so the skin tone and colour must take priority.
I chose this image to illustrate colour cast as it has strong Red, Blue, Green and Yellow content. Also, because the Bracelets are wrapped around a tube of white card, the white areas in shadow should obviously be pure grey, untinted by colour. Notice the lower left, the white paper is virtually identical to the background of this web page which should appear to you as a pure grey (20% Black).
Hat, dress and background are secondary. That certain pinkyness of young skin seems to be absent suggesting that there's not enough blue there meaning it's yellow. The blonde hair looks a little too blonde suggesting yellow also. If we suppose the hat is blue what would happen to it if we added red? It would become magenta - well guess what, look at the crown of the hat, it has that purple look about it that smacks of magenta.
In Photoshop I have opened the Levels Dialogue box, selected the blue channel from the dropdown box and subtracted yellow by dragging the left hand slider control from 0 to 20 (screen grab left).
The result of this you can see below left. The image still has the red content but the blueness of the hat is better.
I then did the same with the red channel but subtracted red by grabbing the right hand slider and dragging that from 255 to 235, the same amount, because I think the original is equally red/yellow. Now, below right, we see a true blue hat because subtracting red adds cyan, we see normal blonde hair and a fairly accurate skin tone.
Compare this with the original above.
One could argue about the skin tone colour. As the lighting is from above the childs face would be picking up blue light coming through the hat. It's things like that one has to watch out for. Don't always assume the subject is lit with white light.
* Watch out for mixed lighting conditions where two or more differing light sources are involved such as daylight and incandescent light (a typical light bulb) which in daylight will appear as yellow. If the white balance on a Digital Camera is set to daylight (or if using daylight colour film) you'll have yellow and white illumination on the subject and the cure for this dilemma is subjective, one must go for one or the other. At Sunrise and Sunset daylight becomes yellow and even red. If a subject is known to be lit with Sunset quality light then it's fair that the image should have a corresponding warmness, a natural look.
The worst scenario is flourescent lighting, like you see in banks and offices, and sodium vapour lighting that one finds in factories. It's an absolute no win situation under these conditions as this light simultaneously flickers and, emits different colours, resulting in the tell-tale greeny-yellow look where everone looks sick. You cannot correct this fault. You can alleviate it somewhat by colour correction and you may have to alter all three colour channels to do so but you'll never get it right!
Let's have a closer look at the lovely lady we saw earlier. Notice the fact that she has Asian features. This would suggest a slightly darker skin tone than caucasian and furthermore, very dark, if not black hair. The shot looks like a straight forward portrait with studio lighting that we can assume is white. The backdrop is of unknown colour. So what's wrong with it?
Well the lips are a dead give away. They look blue/magenta, and her hair looks kind of reddish, but don't be fooled. She's wearing purple lipstick and has had her hair streaked!
As for the amount of correction it's a useful feature of Photoshop that offers a Preview facility. If your monitor is calibrated correctly, and you finally get the image looking right, the amount of change is displayed in the Input levels box above.
Image with yellow removed remains red Final image with red /yellow removed
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