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Colour Constancy
Three examples of colour constancy
C
olour constancy is a name given to the phenomenon that the eye and brain perceive something to be a consistent colour under different lighting conditions. Sometimes this effect can be quite marked, see for example the illusions shown on echalk.co.uk where a neutral grey can look blue under one simulated lighting condition and yellow under another.
The eye is not a camera, or eye and brain combination isn’t. What you are seeing is not necessarily what you see, your vision system is making assumptions and hypotheses all the time.
I am interested in something slightly different from the demonstrations of lighting conditions, as shown in the pictures on this page, which is how colours are being perceived in relation to what is next to them. There has been a lot of work done on this phenomenon using coloured squares, though not much, so far as I know, in real-world situations.
Example 1
T
he photo on the left is in full colour, that on the right is the same photo converted to just shades of grey and of cyan. There is no pink in this picture, or rather there isn’t if you look at the pixels with a colour meter (e.g. the Digital Color Meter that comes free with later versions of Mac IOS). If the eye sees pink, as most people’s will, it is seeing a colour that is technically neutral grey, the areas that are being perceived as pink or magenta are, on the original, areas coloured yellow, orange, light red (i.e. pink) or magenta.
full colour image
grey and cyan colour image
To reiterate, since it can be hard to believe, the photo on the right is solely shades of blue and neutral grey.
Further examination of this photo converted to the print process colours Cyan, Magenta, Yellow and Black can be seen on my Flickr photostream: Colour Constancy Cyan, Colour Constancy Magenta and More Colour Constancy.
Example 2
Night Watch Detail
A
detail from The Night Watch by Rembrandt in the Rijksmuseum, Amsterdam. The blue colour on the shawl of the young woman and the dress of the woman behind her are not, looked at with a colour meter, blue, but grey-green, looked at in isolation you would say more grey than green. This kind of colour:
 
The blue-appearing colour is not a flat colour, each pixel is a different shade, the sample square that I have given being just one of them, but if you look at the blue-appearing areas with a colour meter you will see that all the pixels are a shade of grey-green, not blue. This is an example of colour constancy being used in a painting by an old master, whether he was aware of what he was doing here, we don’t know.
Clearly artists have been taking advantage of this phenomenon for centuries, possibly intuitively. I don’t exactly know how to make use of this systematically when making pictures, though if technical aids to design are to develop as I am sure they must then that hole knowledge should be just temporary.
Example 3
M
y third example is of colour constancy effects using monochrome grey, red and green. This is an interactive demo that can be seen on my page Colour Constancy Overlays.
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The background to this page may be seen on my page Experiments with Colour.

 

 
Dave Collier colour theory page . . . email me . . .