Which colors should I use to indicate higher values in a graph?

Question

Which color should I use to indicate higher intensity, frequency etc.?

I mean, most of the the graphs I've seen use red to indicate high values (intensity, frequency, height) and purple to indicate lower values. Shouldn't it be the opposite considering red has a longer wavelength and purple has a shorter wavelength? Short wavelength means higher frequency.

Examples: intensity, heat, voltage

I know red is more appealing/striking to the eye, but if we were to be technical, shouldn't purple be used?

I'm about to make a graph, so I wanted to know what I am doing.

_{I'm not sure if this is the right place to ask. I'll move the question if it isn't. I just got curious and wanted to know. Also, I'm not familiar with the tags in this site, so I just added color tag. You can edit it if it needs editing.}

Answer 1

Do not use a rainbow of colors to indicate different intensities of values.

To quote Edward Tufte:

Despite our experiences with the spectrum in science textbooks and rainbows, the mind's eye does not readily give an order to ROYGBIV.

(From Tufte's Envisioning Information)

Remember that, despite the rainbow, most people are used to envisioning the spectrum of visible colors as a color wheel. And as a wheel, there is no particular color that means something 'more' or 'less' than any other color. So to use a rainbow puts a rather huge cognitive load on the reader. They have to constantly go back to the legend to cross reference the map with the values.

Instead, focus on different values of one (or, if need be, two) colors.

So instead of the top example, consider the bottom example:

A simple user test that anyone can perform is to take 3 colors from your graph and ask users to put them in order. You'll find that with intensity values, they can always put them in order. But 3 random colors? Well, they're just 3 random colors. They don't necessarily have an intuitive order (some will get ROYGBIV, but not all).

You can certainly use (and should) multiple colors when you need to show different specific values. But if the goal is not so much specific values, but communicating a difference in intensity, a user is going to have a much easier time interpreting the single color with multiple values.

Answer 2

It is not about wavelength. It's about human emotional connection to the colors.

The high indicator is generally red/orange because human nature is to perceive it as vibrant, motion-filled, and dangerous as is fire departments and ambulances.

Blues and purples are perceived as calm and safe. This is why you see hospitals, insurance companies, and emergency services use blues/purples for branding.

Regardless of the actual light wavelength, people will always perceive red as a more extreme value than purple.

Answer 3

Where I live in Australia the Met office (weather) recorded the highest temperature as over about 50 degrees and because Red was the highest representation for the temperature band - they added purple to mark this higher temp on the radar. A key or legend scale might be useful in this situation

Answer 4

This is quite interesting, as I found a scientific tectonic map that seems to use purple as a "higher" value (in this case depth of volcanic activity.).

To me that does not make sense whatsoever, and it seems in general the scientific solutions is, as is pointed out, purple is a "lowest" value. I have read a lot of heat maps, and this is way out.

As for your problem, I think it is safer to use purple as the "lowest" value and red as the highest. I believe that if the scale changes, such as sudden spikes, then purple should be added as the lowest, and keep red as the highest.

Answer 5

This is how I see it:

RED: High value but dangerous, unhealthy
Shades of RED: lower levels of danger (comparable to fire)

GREEN(or BLUE): High value but useful, healthy, good
Shades of GREEN: lower levels of good, pale (comparable to plant life)

MIXED USAGE: RED for danger end and GREEN for safe end.

Going by this design philosophy, it makes sense for me to put PURPLE higher that RED as it is significantly darker. The occurrence of colors like PURPLE in the nature around us is rare and unusual, and it can be used to depict the same i.e a rare and unusual event. And since it blends better with RED, I find it a good contender for depicting a range beyond RED.

Using graphs are a way of giving visual meaning to numbers and hiding the technicality of the raw data, and so its best to choose colors based on human perception. Spectral frequency doesn't strike me as a good basis for a color scale.

Hope this helps :)