Monthly Archives: November 2013

I like my carrots black


On Christmas day of 2009 I posted about the colour of carrots.

I had been watching a Royal Institution Christmas Lecture by Prof Sue Hartley about carrots and why they are orange. She spoke about selective breeding by the Dutch (the first naturally occurring carrots were purple – from Afghanistan – and were later cultivated to be orange). In seeking to find more about this I found myself on the website of the British Carrot Museum. It is seriously worth a visit even if your interest in carrots is tangential.

I was reminded of this today when I came across an article in The Economic Times (India) which reported that the Punjab Agriculture University has developed its first black colour carrot variety (known as ‘punjab black beauty’) which has been recommended for general cultivation in the state. The black carrot is the best alternative to tackle the malnutrition problems of the country because it is overloaded with beneficial anti-oxidants and nutrients. The punjab black beauty is is rich in anthocyanins, phenols, flavonols ß-carotene, calcium, iron, and zinc.

I am also reminded, of course, of the words of the great late Uncle Monty (aka Richard Griffiths): “I think the carrot infinitely more fascinating than the geranium. The carrot has mystery. Flowers are essentially tarts. Prostitutes for the bees. There is a certain je ne sais quoi – oh, so very special – about a firm, young carrot”.

Eat red, eat less


It used to be thought that blue was an appetite suppressant because blue foods are rare and sometimes poisonous. But I have always doubted this and wrote about it near three years ago on this blog. And then nearly two years ago I posted about research from the University of Basel (Switzerland) and the University of Mannheim (Germany) in which it was shown that participants drank less from a red cup than a blue cup and ate less snack food from a red plate than from a blue plate. In other words, the opposite of what was commonly believed. Today I read in CNN about work by Nicola Bruno, a cognitive psychologist from the University of Parma, about his research to measure how much food or hand cream people used when presented on plates of different colours (red, white or blue). The food and hand cream was available to be used freely whilst participants took part in a survey. People ate less food and used less hand cream when either was presented on a red plate. However, the authors note that in their experiment the participants were unaware of the experiment – so it is not so straight forward to extrapolate and conclude that if you buy red plates for home you would eat less. Because then you would be conscious of the idea and it might not work. On the other hand, it might!!

It was nice for me to hear this story and it reminded me of when Nicola came to visit me (when I worked at Keele University) and we published a paper together. That was in about 2000 and I don’t think I have seen him since. Sometimes it isn’t a small world. But it was nice to come across him again anyway.

MRes Colour Communication

colour communication

We’re starting a new programme at Leeds University next September. It’s MRes Colour Communication. This is a one-year Masters programme by research but with a twist. There is a taught component in the first semester to get everyone up to speed to make sure they understand the basics of colour communication. They then explore one aspect of this in their research project and submit a dissertation at the end of the year. Please contact me at my University email of for further information or visit

dog vision

I just read an article in The Daily Mail that says that most people think dogs do not have colour vision. The article then goes on to say that Russian scientists have proved that dogs do have colour vision. It seems to me quite accepted that dogs are dichromats – that is they have two types of light-sensitive cells that contribute to colour vision in their eyes. We – humans – are trichromats because we have three such cells. It turns out that the one that is missing – in dogs – is such that dogs’ colour vision is rather like that of a human who has red-green colour blindness. The image below shows how the spectrum looks to a trichromatic human and a dichromatic dog.


As you can see, dogs can bee blues and yellow but have difficulty discriminating between colours in the red-green part of the spectrum. So I am not sure what the fuss is about with the Daily Mail article. After all, everything in the Daily Mail is true!! See if you don’t believe me.

eyes change colour?


I didn’t realise how sophisticated reindeers are. It turns out they have two layers of fur to help them keep warm, are able to shrink the pads on their hooves to give then better grip, and can detect ultraviolet light which enables them too see in very dim light. And it also turns out that their eyes can change colour in winter so that their vision is more sensitive. Reindeers, like cats, have a reflective layer behind the retina (which is the inside of the eye ball where all the light-sensitive cells are) that helps them to see in dim light. This is why, if you see a cat at night, you might see the eyes shining; you are seeing light being reflected back at you from the cat’s tapetum lucidum (which is the technical term for the layer behind the retina). The light that shines back in most animals with this layer is golden but in reindeer it apparently shifts to blue in the winter. The shift to blue allows more light to be scattered and improves the vision of the animal.

The full paper can be read in the Proceedings of the Royal Society.

good reasons to use colour

I just came across this nice article – – entitled 10 reasons to use color.

The article lists 10 good reasons to use colour in design. Number 10 is using colour for metaphor and taking advantage of the associations that are inherent in phrases such as feeling blue or green with envy. There is no doubt about the meaning in the image below; that the woman is filled with envy.



As some of you may know, I was General Chair of AIC2013 this year. We had a great time in Newcastle and spent a week with over 600 delegates talking about colour. But time moves on and we are approaching 2014. I would therefore like to draw your attention to the next AIC meeting which is in Mexico in October 2014. The theme is colour and culture and the venue – Oaxaca – is stunning. I hope to see you there.

For further details visit

Where is colour mixing?

Imagine that we have three projection lamps at the back of a hall – one has a red filter and so produces a beam of red light, and the other two use filters to produce green and blue beams. We project these onto a white screen and get three circles of light (one, red, one green and one blue). We then move the angles of the projectors so that the circles of light overlap. We get something that looks rather like this:


Where the red and green light overlap we get yellow. We get magenta and cyan for the other two binary mixtures. So,

red + green = yellow

red + blue = magenta

green + blue = cyan

This is called additive colour mixing as I am sure you know. And if we mix all three primaries we can achieve white (or other neutral colours). The primaries could be single wavelengths of light – so we could use a primary at, say, 700 nm (for the red) and one at 450 nm (blue) and one at 530 nm (green). So green light (530 nm) and red light (700 nm) additively mix together and generate yellow. When this happens what is being mixed and where does this mixing take place? Take a few moments to consider this before reading on.

Notice I said that they additively mix to generate yellow – I specifically avoided saying that they mix to generate yellow light. When I sat down with a couple of students last week and asked then what they though they said that the red and green light mixed together to create yellow light and when I pressed them, they went further to say that the yellow light was at about 575 nm.


If we measure the part of the screen that is yellow we would see that we have some light at 700 nm and some at 530 nm. The wavelengths are not mixed; they don’t mix together to generate some third wavelength of light such as 575 nm. So no physical mixing takes place other than – I suppose one could argue – that the red and green lights are mixed in the sense that they are spatially coincident. But that’s not really mixing, for me, and certainly doesn’t even begin to explain why we have the sensation of yellow when we look at these wavelengths together. It also makes me think that additive colour mixing, if it can be said to occur anywhere in particular, occurs in the eye. And I do mean eye, not brain.

Welcome to my blog

I am passionate about sharing my knowledge about colour to anyone who is prepared to listen. I work as a professor of colour science at the University of Leeds, in the School of Design, but I have held academic posts in departments of Chemistry, Physics, Neuroscience, and Engineering. Sounds like a mixed bag, but my interest was colour chemistry, colour physics, colour neuroscience, colour engineering and colour design. You see I have come to believe that colour is the perfect meta-discipline and that to understand colour you need to be able to understand (but not necessarily be an expert in) different fields of knowledge.

One way to use this blog is to just browse through it and dip in here or there. However, another way is to click on one of the categories (that interest you) such as culture, design, fun, and technology and see posts in that area. You can find the categories on the right-hand side of the page if you scroll down.

You can also comment on the blogs. I really like this, even if you disagree with me. Someone once said to me if you put ten colour physicists in a room and ask them a question (presumably about colour physics) you’ll get 10 different answers. Well, I guess not all of you reading this are colour physicists. Given our different interests and backgrounds, and given the complexity of colour, it’s not surprising that we will disagree from time to time. And that is rather the fun part.

If you have a technical question you’d love me to answer you can click on Ask Me and post it there. You can also email me at