How is the number of groups and colours decided for partial grouping of countries?

The decision was to group countries to a small number of groups, say three, according to the values of an indicator achieved by countries. Countries are divided into three groups of approximately equal sizes (terciles): the top third, the middle third and the bottom third. A distinct shade of a selected colour is attached to a group of countries with a similar level of performance. The colour-coding scale graduates from darkest to lightest. The darker shade of purple represents the top third group; the moderate shade represents the middle third; and the lightest shade of purple represents the bottom third of countries. Partial grouping of countries applies to all indicators listed in five dashboards. Sex ratio at birth of Dashboard 2 is an exception—countries are divided into two groups: the natural group (countries with a value between 1.04-1.07, inclusive), which uses darker shading, and the gender-biased group (all other countries), which uses lighter shading. For some very skewed distributions (e.g., total unemployment rate, female to male ratio), the groups differ in sizes greatly. See Technical note 6 at http://hdr.undp.org/sites/default/files/hdr2018_technical_notes.pdf for details