PDI now includes a variety of colour palettes to enhance user experience and help those with Colour Vision Deficiencies

PDI now includes a variety of Colour Palettes to enhance User Experience and help those with Colour Vision Deficiencies

The PDI colour palette consists of twelve components, a foreground one which is black, a background that is an achromatic colour (a shade of grey), and ten colour components for the data to be analysed.

In creating a palette there are some considerations to be taken into account. Firstly, the component colours must be sufficiently different to be easily distinguishable from each other.  Secondly, the background must allow each component colour to be clearly visible, particularly for the visualisation of text. A further consideration is whether the component colours appear distinct from each other by people who are colour vision deficient (colour blind).

Colour palettes can come in three schemes, ‘Qualitative’, ‘Diverging’ or ‘Sequential’.

 

A typical qualitative scheme would be the colours of the rainbow.

 

Qualitative colour scheme

 

In these colour schemes there are no implied magnitude differences, and the hues are used to create the primary visual differences between palette components. This scheme is best suited for representing nominal or categorical data. These palettes are not particularly colour vision deficient friendly unless the individual colours are selected to be differentially perceived.

The default PDI palette consists of basic 8-bit colours (yellow, cyan, light green, purple, blue, brown, orange, green and pink and white) as shown in the figure below.

 

 

Other qualitative palettes consist of: –

  1. “Viridis” colour set shown below. This colour scheme is based on the blue-green pigment, viridian, a hydrated chromium(III) oxide, and is a popular palette used by data scientists.

Viridis colour palette

 

  1. “Munsell” colour set shown below. This colour scheme is based on human subjects’ visual responses to colour where hue, lightness and colour purity are separated into perceptually uniform and independent dimensions.

 

Munsell colour palette

 

  1. Colour vision deficient scheme. ‘Red-green’ type colour vision deficient people perceive the top palette as the one below it.

 

 

There are five other qualitative palettes to choose from.

Diverging schemes typically use two or three hues, for example, blue and red. The scheme puts equal emphasis on mid-range critical values and extremes at both ends of the data range. The critical break in the middle of the scheme is emphasized with light colours, and low and high extremes are with colours that are of contrasting hues. These palettes are also not particularly colour vision deficient friendly.

Below are three typical diverging schemes (there are nine in all).

 

Diverging Red-Blue:

 

Diverging Red-Yellow-Green:

Diverging Red Yellow Green

 

Diverging spectral:

Diverging Spectral Colour Scheme

 

The diverging ‘red-purple’ scheme is a colour vision deficient friendly palette. Normally-sighted people view the palette as the top one, and the one below is how those with the ‘red-green’ colour-blindness condition perceive it. Red-Green colour blindness is known to affect approximately 8% of males of European ethnicity. Very few women are affected as are other ethnic groups and prevalence is much less in other ethnicities.

 

Diverging Red-Purple Colour Scheme

 

Sequential schemes consist of one hue, for example, blue, with the palette components being successive gradations in tones of this hue. These schemes are best suited for ordered data that progress from low to high or vice versa. A typical scheme would be the monochromatic scale in varying gradations in grey tones. These palettes are colour vision deficient friendly as the components differ in tone and not hue.

Monochromatic scheme

Monochromatic Colour Scheme

 

Sequential blue scheme

Sequential Blue Colour Scheme

 

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