I once read that 20th Century research indicated that most people dream in black and white but that modern research reveals that most people dream in colour. The difference is attributed to the fact that in the early 20th century the majority of people spent a long time looking at black and white media. I came across this idea again in The Times today. Sadly, I can’t link to the story because The Times Online is no longer free. Boooo. In fact, I read this in The Review section of The Times today (paper version – I am too mean to pay for the online version). The article stated that a Japanese study published this month by the American Psychological Association found that most people in their sixties dreamt in black and white while the majority of university students (the implication being, I suppose, that these represent a much younger subset of the population) dreamt in rich colour (rich colour – not just colour!). The researchers suggest that younger participants had grown up watching colour TV. However, a psychologist – Ian Wallace – was quoted as saying that he thought this explanation was unlikely sine television represents a tiny part of the visual field and old viewers would have spent far less time watching television that we do today. In fact, the main article was about REM and about how having too much deep REM sleep where we dream could be associated with depression. Anyway, I think it must be very difficult to actually know whether people dream in colour – you could ask them when they wake up, but remembering could, of course, be the result of a false memory.
Why are leaves green? The most obvious answer is that they contain green pigments, the most abundant being chlorophyll and that chlorophyll absorbs the short and long wavelengths in the visible spectrum leaving the middle wavelengths to be reflected and scattered. However, the deeper question is why should chlorophyll absorb in the short and long wavelengths of the visible spectrum when there is more light available in the middle of the spectrum?
The spectral irradiance of sunlight varies with the time of day, the weather conditions, the time of year, and the latitude/longitude. However, I think it would be reasonable to say that by and large, in most situations, the peak irradiance is in the middle of the spectrum (that which we would normally associate with being green and yellow).
So if one assumes that evolution has produced a perfect engineering solution to this aspect of nature in particular then I think one may expect plants to absorb mainly in the middle part of the spectrum (and this would result in the bluish and reddish wavelengths being reflected and a purplish colour).
So why don’t we have a chlorophyll equivalent that is purple? I have come across a number of arguments.
1. One could go further and say that if a plant wanted to be really efficient it would absorb all wavelengths of the visible spectrum and would therefore appear black. So black, rather than purple, would be the perfect engineering solution. Given that most plants are neither black nor purple then I think we can assume that evolution did not find the perfect engineering problem or that the problem is more complex than we think. For example, it could be that a plant that is black would absorb too much light and overheat. Or it could be that chlorophyll evolved from some earlier light-sensitive chemical and that genetic mutations could lead more easily to chlorophyll than to purple or black pigments.
2. Taking this point further, I have heard it suggested that most plants evolved from earlier plants that lived under water and that absorbed mainly short wavelengths of light (long wavelengths – red – cannot penetrate much more than 1 m of water). These earlier cousins of the modern plant would most likely have been brownish. Indeed, if one looks today ay plants in seawater, green plants are only seen on the surface or at very low depths. So the ancestor of chlorophyll could have been a brown pigment which mutated into green chlorophyll more easily than it could have mutated into a purple pigment.
3. I have also come across the ‘early purple earth’ hypothesis. This suggests that originally most plant life on land was indeed optimally purple and that chlorophyll absorbed to take advantage of those wavelengths that were not already being gobbled up by the dominant species. Subsequently, chlorophyll proved more successful than its purple companion.
4. It could be argued that optimally absorbing light (and being purple) is not the most important thing and that there are other aspects of the problem that are more important. Green chlorophyll could be the optimal solution to this more complex problem.
In short, the real answer is … I don’t know. I am not overwhelmingly convinced by any of the above arguments.
If you enjoyed this post you may like to look at my special christmas post on carrots and why they are orange.
Many studies have been carried out over the last 50-100 years to look at which colours people like and which they don’t like. Although there is variability between individuals (not everyone likes the same colours) there is surprising consistency when the results of lots of different studies are compared. In short people like blues and greens and don’t like yellows and (to a lesser extent) reds. The hue parameter is probably the most important but brightness and colourfulness also affect colour preference. People tend to like brighter and more colourful colours than darker and less colourful ones. Just for fun, I have been running my own survey on this web site. You can still add your two-penneth worth if you like – please go to http://colourware.wordpress.com/2011/02/22/favourite-colour-poll/. Interestingly, my fun survey is also in broad agreement with all those previously published experiments. I found that people’s preferences were:
I am not sure what practical application there could be in knowing which colours are more popular. For example, my favourite colour is red but I probably wouldn’t want to buy a red coat. Though on average most people really like blue, this doesn’t mean it would be sensible to make a product blue without consideration of many other factors. In design, colour is almost always in context and that context makes all the difference in the world.
More interesting though is recent research I have read which proposes a reason why there is individual variation in colour preference. According to this idea, we like those colours that remind us of things that we like (we like blue skies and green grass). It could explain dark yellows and oranges are particularly unpopular; these colours are normally associated with some rather unsavoury things (dark orange is the colour of poo and dark yellow the colour of vomit). Further, if people have a strong affiliation with an idea/concept that is strongly associated with a colour, then they may have some preference bias towards that colour. It makes me think – I am a hug fan of Manchester United and red is my favourite colour; but do I like red because I like Manchester United or do I like Manchester United because I like red? I am too old to remember which came first.