Design Pattern: RM(8) Colour

Context

A set of incoming alarms has been registered. Alarms have different priorities, they can be registered at different locations, and different elements can trigger them. Accordingly, the human operator would like to characterize these alarms in order to spot the most important ones for the current status of the controlled process.
Problem

The human operator needs to characterize the nature of alarm activations according to different dimensions in order to spot the most important ones.

Solution

Code categories of alarms by using colours. The number of colours should be kept to seven or less, and be consistently applied. Seven corresponds to the number of items that can be generally being kept in short term memory. Once colours are assigned a specific use or meaning, no other colour should be used for the same purpose. The seven colour codes do not limit the coding of other information separate from the category information. When a user must distinguish rapidly among several discrete categories of data, a unique colour should be used to display the data in each category. When the relative rather than the absolute values of a variable are important, gradual colour changes, as a tonal code, should be used to show the relative values of a single variable. Finally, brighter and/or more saturated colours should be used when it is necessary to draw a user's attention to critical data.
Known uses
Fig1.Light blue for low-priority alarms, orange-yellow for high-priority alarms, red for emergency-priority alarms (and pale red for acknowledged emergency-priority alarms, magenta for off-normal operating conditions).
Fig2.Yellow for high-priority alarms and red for emergency-priority alarms
Rationale

Coding based on the use of colour is an important means for representing categorical information in displays (Spence, R. (2007). Information visualization: Design for interaction (2nd Edition ed.) Pearson-Prentice Hall.)
Relations