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: