Colour coding by day - any conventions?

Question

I'm working on a map showing on which days garbage is collected in different areas. That means it's pretty tempting to colour code each zone with a different colour representing a day of the week.

Are there any conventions to follow? Is 7 colours too many?

Here's what it looks like at the moment:

My colours so far follow the rainbow from Monday to Friday (red, yellow, green, blue, purple) then light grey and dark gray for Saturday and Sunday.

Here are the numerical values in the image, for interest. (CartoCSS, Tilemill.)

@s: 60%;
@l: 70%;
@monday:    hsl(  0, @s, @l);
@tuesday:   hsl( 60, @s, @l);
@wednesday: hsl(110, @s, @l);
@thursday:  hsl(200, @s, @l);
@friday:    hsl(250, @s, 80%);
@saturday:  hsl(  0, 0,  70%);
@sunday:    hsl(  0, 0,  70%);

So the weekdays are all the same luminosity and saturation except Friday, which I found seemed too dark, for some reason. I originally spaced the hues equally around the spectrum, but found that there were two blues too similar, and several of the colours just weren't attractive.

Answer 1

Given that you have exactly seven classes (do they really pick up on Sundays?), and that they're categorical (not quantitative) classes, I'd say that having seven different hues and yet similar intensities and saturation levels is a good way to go. You could probably even squeeze, say, orange and indigo/violet_‡ in there, in place of the grays.

While you could use near regular spacing along/around the hue scale, ultimately, you should rely on actual perceived differences in color to guide your choices (and you seem to have already experimented with this).

Given that some folk are color blind, or that some might be seeing a monochrome (gray-level) reproduction of your map, consider also combining a unique, subtle pattern or texture with each color, to aid in differentiation.

_{‡ whichever one is not like the purple you already have}

Answer 2

7 colors is not too many. My go-to resource for at least getting a start on a color scheme is ColorBrewer, their tool will allow you to pick the number of data classes you have, the nature of the data (quantitative, qualitative), and a color scheme (dark, pastels, etc), and will give you a preview of how your data will look and the color codes you'd need to implement the scheme. Go there, choose a Qualitative color scheme, and play around.

I wouldn't use a shade of gray or black unless it's to indicate the absence of trash pickup on that day. If trash is picked up every day, use a 7-color scheme. If it's picked up on weekdays only, use a 5-color scheme during the week and gray on the weekends to indicate no pickup on those days.

Answer 3

It is not clear whether “day of week” has as categorical/nominal or an ordinal scale. Since your colors “follow the rainbow from Monday to Friday” I assume that the order/sequence of the days is important for your application/analysis. Therefore, the following applies only if “day of week” has a ordinal scale in your application/analysis:

I am not aware of a cartographic convention for day color. But 7 colors (“rainbow”) are too much because a sequence is not recognizable.

In the first step the scale level has to be determined. In my opinion, the data are ordinal scaled (in opposite to categorical/nominal, interval, ratio).

With ordinal scaled data the color must be visually ordered (from Monday to Sunday).

In a HSV color model you have three options to adjust the color:

• hue (green, blue, red, …)
• saturation (amount of gray)
• value (brightness)

In your map you used 7 completely different hues to vary the color (red, yellow, green, blue, purple, light grey, dark gray). These are too many. A sequence is not recognizable.

Here is a good explanation how to build a color sequence (http://colorusage.arc.nasa.gov):

With ordinal data the labeling colors of the graphic elements must be not only discriminable and identifiable, but also visually ordered. The color assignments have to express the sequential relationships among the graphic elements. This can be achieved with a hue sequence, a saturation sequence, a lightness sequence, or some combination of the dimensions. Monopolar hue sequences can be obtained by mixtures of varying amounts of two non-opponent hues, i.e., some pair other than red/green or yellow/blue. Saturation and lightness are naturally visually ordered. Combinations of saturation and lightness work well (see example below). For bipolar ordinal labeling a combination of saturation and lightness in two hues works well.

Here are quick examples showing a better color sequence than your "rainbow" colors:

Example: gradient between 2 hues (yellow to orange):

Example: 6 different saturations/values:

Example: 6 different values: