Colour

Colour became more widely used on maps in the mid 19th century, when lithographic printing made it faster and cheaper to incorporate. Today, it is an important part of any mapmaker’s toolkit, and has a big impact on the overall legibility and aesthetics of a map.

Pioneering cartographers such as Eduard Imhof followed a set of guidelines when using colour, which made their maps as legible as possible and gave them the iconic cartographic styles which have stood the test of time. These included only using strong, deeply saturated colours in small areas of extremes, to avoid placing them next to each other in large areas. Imhof also noted the importance of base colours as they give way to the smaller, brighter areas. This is why we often see lighter shades such as grey and beige used on cartographic basemaps.

Eduard Imhof’s hand-painted map of Mt. Everest from his book ‘Schweizerischer Mittelschulatlas’, 13th ed. Zurich: Konferenz der kantonalen Erziehungsdirektoren, 1962

Eduard Imhof’s pioneering use of colour to represent terrain makes his maps both easy to read and beautiful to look at.

Different colour schemes and when to use them

You can adjust the colours you use on your maps by altering the hue (what we typically know as the name of the colour e.g. orange or green), the saturation (how vivid the colour is), and the lightness (how close to black or white the colour is).

The content of your map will also determine the best type of colour scheme to use. There are three main colour schemes: qualitative, sequential, and diverging.

• Qualitative colour schemes use colours of different hues which have no visual relationship to one another. You should use this type of colour scheme when displaying discrete data that is defined by specific categories which don't relate to one another. This could be something like building use where there are residential, commercial, or mixed uses – or to map locations of different ethnicity groups.

• Sequential colour schemes use one hue, with varying amounts of saturation and lightness. This colour scheme works best with maps that are showing information on a sliding scale, for example house prices, or pollution levels which go from low to high.

• Diverging colour schemes use two hues which are more saturated at the two ends of the colour spectrum, which will then blend into each other in the middle with a more muted shade. They are most suitable for displaying information on your map which has some data that is at the extreme ends of the scale, but most of the data sits around the average middle point. This could be things like information on temperatures in a place, or the amount of rainfall.

When using any of the above types of colour schemes, it's good to think about the appropriate number of classes you're categorising data into. If this gets too large, it will become difficult to distinguish between the many colours and shades, meaning your map is harder to understand.

It is usually a good idea when creating topographic maps, to keep in mind the typical associations of colours on maps and what they represent – as this will help your user quickly interpret your map. For example, using green hues to signify natural features, and blues for water.

You can access the GDV toolkit through GitHub where you will find GDV colour palettes and a range of useful tools for using these colours within a GIS.

Other considerations when using colour on maps

There are a number of important considerations to bear in mind when using colour on maps, as if colour is not used properly, it can have a negative effect and confuse the viewer – or even make them not want to use your maps.

Colour Vision Deficiency (CVD)

Often known as colour blindness, this is a visual condition which impacts people's ability to differentiate between different colours – and so can cause difficulties with using maps which so often heavily rely on colour to be understood.

CVD affects around 8% of men and 0.8% of women in the UK, and it is likely that maps you make will be seen by a person with CVD at some point. There are a few different varieties to be aware of:

• Deuteranopia (aka red-green colour blindness, where these are the hues which are hard to tell apart for people with this condition)

• Tritanopia (aka blue-yellow colour blindness, where these are the hues which are hard to tell apart for people with this condition)

• Achromatopsia (aka monochrome vision, where the person will see in greyscale)

There are accommodations which can be made to ensure your maps are more accessible for people with CVD. Using resources like Colour Brewer or Adobe Color to check your colour schemes or take one of their pre made ones are great ways to ensure accessibility. It's also a good idea not to use colours with similar saturation and lightness, as these won't have as much contrast from one another. Adding text labels to your map to call out important areas is another great way to take the reliance from being solely on colour when interpreting your map.

We have created some custom stylesheets for people with CVD for our OS Open Zoomstack product – you can download them and get started here.

More information about how to make your data visualisation accessible can be found here.

Colour systems

It's important to consider the format of your map when choosing which colour system to use. There are 2 colour systems:

• Additive colour systems relate to anything that emits or radiates light – such as screens. They use Red, green, and blue hues combines to represent the colour spectrum, with different wavelengths of light creating each of these different colours. The more light which is added, the brighter the colour becomes and so when all the colours are combined they create white.

• Subtractive colour systems work on the basis of reflected light, and so apply to colour in printed mediums. Instead of pushing more light out, the way different pigments reflect different wavelengths of light determines the colour we see. It uses 3 main colours – cyan, magenta, and yellow – but here white is the absence of colour, and black is the combination. Interestingly though, when all these colours are combined we don't get a 'true' black shade, instead a dark muddy brown hue. Because of this, a fourth pigment is added to create black, called key (giving CMYK).

So, if you are making a map to be displayed on a computer or another type of screen you should use an RGB colour system. If your map if going to be printed off as a physical copy, make sure to use a CMYK colour system so that your colours are displayed correctly when printed.

Connotations of colour

It can feel overwhelming when starting a new map, to know how to choose colours to represent your different features. Familiarity is important, and so using greens for vegetation, blues for water, and browns for urban areas or buildings is a good place to start.

It is in our human nature to have associations with different colours. For example, we often think of danger or stopping when we see red hues, whereas blue can invoke feelings of calmness and peace. Maps depicting disease outbreaks or warzones often purposefully don’t use red for this reason, as it can create an additional sense of panic.

Going back to the principles of Eduard Imhof, try to use more subdued, lighter tones for features on your map which you want to appear in the background – and brighter, more saturated hues for anything you want to draw the user’s eye to first.

Colour on maps can seem like a complicated topic – and there is a huge amount of scientific research into it. But hopefully, by following some steps and taking care to use the appropriate colour schemes for your map, you should be making legible and beautiful maps in no time!