<p>This is a tricky subject indeed, and the blue/purple confusion is not the only implication. <br>
We have mainly two systems of colour when reproducing reality, the additive system that the digital cameras use, and the subtractive system, in the colour film age of the last century the mostly used in photography, and which is inevitably the only one suitable for printing out photos. <br>
Both systems are represented by the "colour wheel", but those "wheels" are different of course. Our additive system has three colours, red, green and blue, while the subtractive system has the complementary colours of those, cyan, magenta and yellow. Mixing the additive colours additively will produce those same subtractive colours, and mixing the subtractive colours subtractively will produce the additive colours. <br>
But, and this is a strong <strong>but</strong>, as the additive colours are made up by mixing subtractive colours subtractively, they actually do not exist in the subtractive system, and likewise, the subtractive colours do not exist in the additive system - as physical true wavelengths, frequencies of light, producing those sensations of colour in our visual system, in the cortex of our brain. Because the "colours" we capture on our chip or film, are not colours. They are frequencies of optical energy, electromagnetic energy emitted from an energy source, and they do not have any colour at all, they are just energy. The colour is a figment of our imagination, it exists solely in our brain. We imagine the colour, it is our response to those energies, and as trichromates, beings sensitive to three "colours", our perception of the colour of light that reaches our vision, is always registered in our brain as a mix of those three "colours", the sensitivities of our three colour sensitive cones in the retina. <br>
It might be a bit difficult to fathom, that the strongly saturated yellow that we see in some flowers is a colour that cannot be saturated in the colour system that we use, neither in the eye, nor in digital photography. In digital photography, yellow is a colour that cannot be saturated, because it must be composed of the two colours red and green. Saturation means absense of other colours, and yellow is entirely made of other colours, there is no yellow in it, in our digital image, only red and green, while blue is absent. <br>
The colour space we use is triangular in shape, and the triangle in the diagram of the colour space for additive colour has its extreme points in other directions than the triangle for subtractive colour. In our additive system, violet, as the violet of short wave light, does not exist, but purple does, as a mixture of red and blue. If the red filtered sensels in the camera completely blocks short wave light, they cannot register violet; only the blue sensels can. So if the red channel is present in the purple representation of a violet tone, it can have three explanations: </p>
<ol>
<li>The colour actually is purple, a mix of blue and red</li>
<li>The red filters in the colour filter array has two humps in their spectral response, one for red, another for violet</li>
<li>The software introduces red where there is no red, but only a saturated blue</li>
</ol>
<p>The CRT. LCD or LED computer screen is strictly additive and cannot produce violet, to display purple, it must use both blue and red. <br>
I don't have the answer to which of the three possibilities is valid, although there's evidence of much confusion. I cannot find on the web any evidence of red filters with violet sensitivity, although possibly the red-sensitive cones in our eyes might have a raised sensitivity at the far end of violet which causes us to assess violet as similar to purple (purple being the mix of red and blue). <br>
I am fully aware that this subject is very confusing to many people, but I have tried to explain it without adding to the confusion. The "colour wheel" is cyclic, but the spectrum is straight and has two ends. The colour wheel joins the ends of the spectrum, making it cyclic, and causing violet to fall adjacent to red in the cycle. Our additive display media however will produce the colour purple to imitate violet by mixing red with blue. <br>
I'm aware that this does not answer the question why blue turns purple or why purple turns blue in images, but I think it adds to understanding the processes that display colour in images, and the problems in designing filters and software for capturing and representing colour, as well as displaying it in different media. The problem in printing is similar, but reversed. In printing, we can indeed produce a saturated yellow, but not blue, in a colour photo. The same when we capture on film, there is no blue in colour film, only cyan, magenta and yellow. When the printer needs a solid blue, as for example to a logotype, printing is done with solid blue separately, as it cannot be made to exact specification with the tricolour CMYK system. </p>
