For many many reasons, including readability, accessibility etc., good contrast is essential for anything you need to read… none of us is getting any younger. Black on white (or vice versa), even dark grey on off-white, is good. But light grey on medium grey is not (I’m looking at you, all the 20-something designers who seem to think that 50 shades of grey is cool for apps, software, and websites. It’s not.).
Similarly for colours—not only do you need to use contrasting colours, but you also need to make sure, where possible, that you are using colours that can be readily distinguished by those with limited vision or some form of colourblindness, and colours that don’t ‘flare’ or clash when you try to read them (e.g. don’t use red text on a green background or vice versa).
In the environmental reports I edit, typically there are several maps. One of my editing tasks is to check the spelling of the place names and any other text on the map, and also to check the legend to make sure that all the colours or symbols used in the legend accurately reflect what’s on the map. Because many of the companies I work for have large GIS departments, the maps are mostly very professional and accurate and it only takes me a minute or two to check them. But sometimes something is wrong and I have to query it.
In a report I worked on recently, they had included many maps. All were made by the same GIS team, and nearly all had reasonable or good contrast in the colours used on the map (as reflected in the legend). For example:
But then there was this:
Even someone with PERFECT vision would have a hard time distinguishing the different areas of this map, as indicated by this legend. And it would likely be impossible for someone who had blue/yellow colourblindness.
I made a note to the author to talk to the GIS people and get them to change this map so it had better colour contrast—there are 16+ million colours they could have chosen from, yet they chose a set of blue/greys and a set of murky yellows that have little to no contrast between them.
(Note: In the document, this legend is MUCH smaller than is shown in this screenshot, making it even harder to differentiate the colours.)