The picture for this article is a block of colour with no alternate text or title. Assuming you can see it, think about what you would call it before you read on.

If you described the colour above to someone else it’s likely they would imagine something quite different. You could describe it as lavender, lilac, mauve or pale purple but in each case there’s no universal meaning for these words.

If you’re more technically minded you could sample the colour to discover its hex value is #9B90C8 but if someone else used that value to create an identical image there’s no guarantee they would see the same thing or give it the same name. This is because our perception of colour is relative, subjective and learned.

Firstly, each colour we perceive is relative to the other colours that surround it. The most powerful example of this is the illusion of yellow that’s created when you move brown into the shade. Brown doesn’t appear in the rainbow and we only perceive brown objects based on the brightness or darkness of other objects in the vicinity.

Secondly, each colour we perceive is subjective because each pair of eyes is different. Some people are colour blind which suggests a disability, although research has found there are some advantages such as being able to spot certain types of camouflage. Some people can see ultraviolet and it’s also been suggested others have four sets of colour receptors instead of the usual three.

Finally, each time we perceive a colour we relate it to what we’ve learned, some of it useful and some of it less so. We learn the names of seven colours of the rainbow where in reality a full spectrum of light has as many colours as you choose to name. Many of us learn that purple and violet are synonymous. Many of us leave school with the idea of red, yellow, blue and green being the four primary colours.

However much you learn, it’s not possible to recall the name of a colour in the same way that you can the name of a musical note. There is no equivalent in vision to perfect pitch. So if I were to reveal that Pantone call the block of colour on this page 16-3823 Violet Tulip, their colour of the year 2004, you might like to remember it but you could never be certain you were seeing the same colour again.

Perhaps one thing we can agree on is black as it’s something we all experience when no visible light reaches the eye. But even here there are differences. I live near Sean Kanavan, a blind guerilla gardener who plants flowers in his street. As he went blind later in life he remembers colour and always likes to hear how his hollyhocks have turned out. But to someone born blind, blackness and nothingness are quite different as Richard Gregory explains in his book Eye and Brain:

The sensation given to us by the absence of light is blackness; but to the blind it, it is nothingness. We come nearest to picturing the world of the blind, who have no brightness and no black, by thinking of the region behind our heads. We do not experience blackness behind us: we experience nothing, and this is very different from blackness.

Most of us can perceive a spectrum of colours ranging from red through to violet, the visible spectrum being just one part of the wider electromagnetic spectrum ranging from radio waves through to gamma rays. As radio waves and light are essentially two different forms of electromagnetic radiation I wanted to consider what it would be like if our eyes were tuned differently to see the waves of Radio 4.

Some insects such as bees see beyond the violet at the end of our visible spectrum to pick up ultraviolet light. Similarly snakes see beyond the red at the other end of our spectrum to pick up infrared. The world is awash with many kinds of electromagnetic radiation sailing by at the speed of light which machines and some creatures can detect but most of which we ignore.

In simplified terms electromagnetic radiation is made up of tiny packets called photons which oscillate in waves to carry energy from one place to another. The difference between each type of radiation is down to the length of the waves and the frequency of the oscillations.

Each colour of the rainbow has a different frequency just as each radio station has a different frequency. In analogue broadcasting each station has a carrier wave which is modified or modulated according to the content being broadcast. The frequency of the carrier wave is the number you use to tune your radio.

With amplitude modulation (AM) the strength of the signal being transmitted is varied by the audio input. When there’s a period of silence the signal stops, and during peaks of speech or music the signal is at its strongest. If you could see the signal from an AM radio mast it would look like a pulsing light, each station’s distinct frequency appearing a different colour.

With frequency modulation (FM) the strength of the signal is constant but the frequency is shifted a tiny amount by the audio input. When there’s a period of silence there’s a fixed signal, and speech or music makes the frequency wobble. If you could see the signal from an FM radio mast it would look like a light with constant brightness with the colour subtly shifting up and down the spectrum.

I sought the help of my brother Richard, an electronics engineer, to demonstrate the changing colours of FM radio in real time. To see FM requires three steps. Firstly the carrier frequency needs to be shifted to one within our visible range; secondly the modulation needs to be increased so the colour changes are clearly perceptible; thirdly the signal needs to be looked at in blocks so it doesn’t appear as a blur.

My brother wrote some software which takes an audio input and modulates a signal in the same way as an FM transmitter although in this case it outputs colour to a screen not a signal to a radio mast. For simplicity we centred the output on green, extended the modulation to include the full colour range of a computer monitor from red to blue, and looked at 0.5 second blocks at a time.

The image for this article shows a fragment of Sailing By which is broadcast every night on Radio 4 at around 0045 UK time immediately before the late shipping forecast. I find it soothing to imagine that while most people are sleeping the UK is being illuminated by the wobbling colours of this classic tune. If only we could see it.

The colour violet is intriguing as it’s one we don’t see every day. It’s outside the colour range of television screens and computer monitors, and it can’t be faithfully reproduced by standard colour printing processes. The reason for this is complex but I’ll attempt to explain it in simple terms in one page.

In the 1670s Isaac Newton explained why white light from the sun can be split into a spectrum of colours using a prism. Although light is a continuous spectrum, Newton named seven colours of the rainbow as it fitted nicely with the seven notes in a western musical scale and the seven known planets at the time. This is why we learn the colours of the rainbow as red, orange, yellow, green, blue, indigo and violet but in fact there are as many as you choose to name.

Most humans have three sets of colour receptors each sensitive to a band of colours with peaks in sensitivity corresponding roughly to red, green and blue light. When we perceive yellow light in the rainbow both our red and green receptors are stimulated as the pure yellow falls between them on the spectrum. To a human, red and green light combined or the pure yellow of the spectrum appear to be the same.

By combining red and green with blue light it’s possible to create the perception of most everyday colours. This process is known as the RGB colour model. The full range of possible colours depends on the exact hues of red, green and blue that are chosen. Historically it was hard to produce the really deep red and vivid blue-violet light needed at sufficient intensity to produce a wider range of colours. As an example the sRGB colour standard developed by HP and Microsoft encompasses less than 50% of the colours visible to most humans.

The difference between violet and purple causes some confusion as they are conceptually very different but the names are often used synonymously. In physical terms colour can be considered a linear spectrum but our brains perceive colour more like a wheel. Between red and violet is a wedge of purples combined by mixing these two extremes. Purples are extra-spectral colours which means you won’t find them in the rainbow. They are, if you like, pigments of your imagination.

Violet, the pure spectral colour on the inside edge of the rainbow as opposed to the purple in your brain, is beyond the blue of an RGB monitor so it is impossible for me to recreate it for you on this page. True violet occurs rarely in nature but when you do see it and try to photograph it with a digital camera you normally end up with blue. This is because most cameras are based on the same RGB model.

I used a specialist dichroic filter to produce violet light, not the purple light you can see from a screen or the effects of ultraviolet light you see in clubs, just an intense, pure version of the visible violet you see in the rainbow. It’s a truly beautiful colour. If you want to experience violet and don’t want to wait for the next rainy day remember there are many ways to make your own rainbow.

• The colour violet
• What Radio 4 would look like if you could see it
• Other interesting stuff about colour

If you have any comments or questions please contact me via Twitter.