Colour is worthy of great study as a topic on its own
When creating a map, we always ask if there is a company brand identity style guide, as this helps immensely to work out the style of the map including its colours. The colourways for a particular map not only have to work with the company brand, but work well together on the page. Much of the art in cartography is in ensuring that the colour combinations work well together, convey the right message and are easily understandable by the viewer or user.
It’s all about perception
The eye is a fickle and unique organ of the body, which actually ‘sees’ the world upside down. It is your brain that flips the image back the correct way. Therefore, everything you see is based on your individual perception and your cultural references. When you see the sea on a map, what colour would you recognise as the sea? Would you need the key to tell you that the image in front of you has the sea in it? Or do other cartographic elements help you to understand that an image represents the sea? – such as fish, boats etc – or did you recognise the sea solely from the colour? 
More than likely it is a combination of all of these elements which provides the viewer or user with clues and then you bring the ‘aha’ moment from your particular colour cultural reference. This is why it is really important to work with the end user that you are working for in cartography. The cultural, religious, societal, historic and personal viewpoint can change the meaning of each colour viewed on a map.
Add to this the further complication that each person sees colours differently. Colours can have shadows, highlights, reflections and patterns in them making the perception of colour subjective. What one person sees as green, may look to someone else as more bluey-green. As cartographers it is important to remember what the user sees is more important than the particular colour we have chosen. Turquoise is one of those tricky colours. Some people see it as more green and others as more blue. Try not to use turquoise in a map if also using blue or green for this reason. Try it for yourself around the office and ask if the colour turquoise is more green or more blue, you’re sure to get a difference of opinion.
Overcoming the first issue of perception is the main challenge. Drilling down into how people perceive colour means examining where the users will be viewing the colour. Try viewing the same turquoise colour indoors (lighting) and outdoors (under daylight), this will change the perception of the colour. Our eyes adjust to different light levels much quicker than a camera lens and we can see in much less light levels.
The photographs below show three images in different light conditions and how the colours change:
The quality of the paper (stock) makes a difference to colour also. Plain paper or high-gloss? Both will change the users perception of the same colour. High-gloss is highly reflective and changeable in the light, this makes it attractive to look at, but can make it hard to see solid colours in a map key for example.
It’s the same on-screen. The colour you see is because of your monitor. Even monitors which are colour calibrated still differ from machine to machine and from one lighting situation to another. One colour represented on a monitor or screen is not indicative of all the variety of devices that it might be seen on. The light from your computer or device is made up of RGB (Red, Green, Blue) which combined produces white. The absence of colour makes the screen black. This makes colours on screen seem brighter, whiter and different from the printed page. Be wary when printing greens and purples from RGB on a map as these are the colours that can change dramatically from screen colours on a map.
Many years ago computers displayed 256 colours, the major computer platform developers did not use all of the same 256 colours, hence colours looked different on various machines. A set of 216 colours were selected which are called ‘web-safe’ colours. The reason for less colours is that only 216 colours will display exactly the same on all computers.
For making thematic maps online, you can use ColourBrewer with colour-tested combinations. 
Not all inks are created equal
Having examined the perception of the user and the situation the user is in, you would think the user could see the same colour. Well, not necessarily when printing the maps. Just because the map has been converted from RGB to CMYK (Cyan, Magenta, Yellow and Black) for your printer, doesn’t mean that this will be printed faithfully from your computer to the printer. If you think how many different types of computer there are and how many different types of printers and ink there are, the colour you think you are printing may not be the colour you are actually getting on the printed page. The way to resolve the colour issue is by testing the set-up to see if you get the results you were expecting and that they meet the users needs.
This is why the international colour standard Pantone is so fantastic and why it’s revered around the world. 
Pantone Colour System
Pantone is a colour system which began in the 1950’s. A commercial printing company hired a graduate Lawrence Herbert to use his chemistry knowledge to systemise and simplify the company’s stock of pigments and production of coloured inks. By 1962, Herbert was running the ink and printing division. He then bought the company’s technological assets and renamed them Pantone. When looking for colour-matching across continents, using the Pantone Matching System (PMS) for maps is essential.
Pantone not only does colour matching for print-to-paper, but dyed fabric, plastic chips and colour matching between all of these variables. It is a truly stunning achievement of colour-matching that can be used around the world and one of the fundamentals when creating maps.
Combined colour distortion
Some colour combinations create ‘auras’ around them, making them uncomfortable to view. Conversely, some colours can camouflage one another making it difficult to distinguish between them. This has long been part of the ‘Magic Eye’ poster series.
Optical illusions are fun, but they also point out the importance of why one colour should be studied next to another colour – not be used or tested in isolation.
Testing colour combinations
This is the gold standard of colour theories; experimenting with colour is the only way to find a good combination . In a laboratory or in a studio particular colours may really work. They may work with the client’s brand identity style guidelines, work with the cultural understanding of the user. They may work with RGB and CMYK as you’ve chosen Pantone colours to get the most consistent colour results. However, if you haven’t put these colours next to each other and tested them, then you have failed the colour challenge as a cartographer.
This is a map created by Clear Mapping Co using the client’s style guide and colour combination:
We created a colourful map with a key that highlights quite dry information and made it more interesting, by using map colour conventions (western/mobile) in a surprising way.
If you are looking to make better maps, we can advise you on the best way to represent data to your audience. We ensure that our maps are as sustainable and as accessible as possible.
We can provide the consultancy to set you on the right path.
Get in touch: +44 (0) 1326 337072 or by email firstname.lastname@example.org.
 Image Music Text 27 Feb 1987 by Roland Barthes
 The Complete Color Harmony, Pantone Edition: Expert Color Information for Professional Results by Leatrice Eiseman (Flexibound)
 Interaction of Color: New Complete Editionby Josef Albers, Nicholas Fox Weber (Paperback)