The psychology of colour is fixed in us from an early age and businesses have a long-standing obsession with traffic light inspired data visualization. But how effective are these red-green dashboards when viewed by someone who can’t differentiate between these colours?
Data visualization good practice should include accessibility of design.
The reason traffic lights are red and green
Red has long been used to signal danger, but why? Red has the longest wavelength of any colour on the visible spectrum, which means it can be seen from a greater distance than other colours. This is perfect for a driver who needs as much notice as possible to stop.
Image: Red has a longer wavelength than any other colour on the visible spectrum.
Green is located on the opposite side of the colour wheel to red. Used together they show the most contrast between each other than if they were paired with other colours. Green has a mid-length wavelength on the visible spectrum so can also be seen from a relative distance.
Image: Red and green are located opposite each other on the colour wheel.
Using this logic it makes sense to design a traffic signalling system using these colours. But, what about the 5% of the population who can’t differentiate between red and green?
Colour blindness affects approximately 1 in 12 males and 1 in 200 females. Chances are you know a few people who have a colour vision deficiency (CVD). The high contrast seen between red and green with normal vision is not present for someone with the most common type of CVD, as they are unable to differentiate between these two colours.
Image: How red and green can be viewed by different people.
How to make data visualizations accessible for CVD
Colouring a data visualization should always be done strategically – leave the rainbows in the sky! Colour is one of the most powerful preattentive attributes when used correctly. A data visualization can still benefit from the psychology of red (stop) and green (go) but only if you understand clearly the message you want to communicate.
For example, the two graphs below show the same data but their narratives are very different. The graphs wouldn’t be used together but at the discretion of the report writer, depending on what they wanted to communicate.
Image: Graphs showing elements of positive or negative storytelling.
Tip: Red and green are fine to use independently of each other. The CVD problem occurs when the colours are used together as the only way to understand data.
If you must use red and green together
Ideally, I wouldn’t recommend using red and green together but there will always be situations when you can’t avoid it. We can learn from the reasons traffic lights still work and apply these to our data visualizations.
1. Relative position
There is a consistent order to the position of the red, yellow, and green lights (depending on where in the world you are). This way of reading traffic lights is not as intuitive as colour differences and may require a slightly longer reaction time, especially at night.
Image: Different traffic light layouts around the world.
Tip: Try to not make colour the only way you differentiate between data points. Position, shape, and size are all good design principles to use in addition to colour, for your chart to be accessible to most audiences.
Image: A traffic light in Canada with specially shaped lights helps assist people with CVD.
Tip: I like to design data viz prototypes in greyscale only. If it works in black and white, it will also work in colour - which can be added later.
2. Shade of green
Traffic light green isn’t actually green-green (or grass-green). Its wavelength pulls it slightly towards the yellow or blue (depending on where in the world you are) parts of the visible spectrum. This doesn’t give it a lot of difference for someone with CVD but makes it slightly more accessible.
Tip: Try to push your ‘green’ to more yellow-green or blue-green parts of the colour wheel (remember painting as a kid... blue and yellow make green!).
Image: Adding blue or yellow to your green will make it more accessible when used alongside red.
Tip: If you’re working with RGB values, increasing the ‘G’ value of your green will lighten and intensify your colour. This can give enough contrast against a duller red for CVD viewers. Be careful when printing as high ‘G’ values won’t be viewed the same on paper as on screen.
Image: Increasing the 'G' RGB value of your green will make it more accessible when used alongside red.
Tip: This website created by Susie Lu and Elijah Meeks is a great tool to check how your colours are viewed by someone with CVD.
Change your thinking on data visualization design
Many fields are designing for accessibility but I feel the business data visualization world is slow to respond to this.
If you are responsible for communicating with data please be aware of the people who don’t view the world the same way you do.