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

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.

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.