Color Spaces, Palettes, and Perception

There is Gold at the End of the Rainbow

Why is the Jet palette so bad?

Name the colors of the rainbow in the correct order...

1. The order is not intuitive to everyone.

2. Categorization is intuitive but unwanted and not consistent.

A Jet-like palette in action

3. The lightness is not linear.

Color Spaces

  • Colors are mixable.

  • Any 3 colors can span a color space.

  • What is the most useful coordinate system?

Primary Color Spaces

  • Color Matching Functions (CMF):
    spectral colors <-> color space coordinates.

  • Color spaces allow only either + or -

  • Any primary colors have partly negative CMF.

RGB color space

  • RGB covers most of the spectrum accurately.

  • 'Darker' primaries (RGB) for + spaces,
    'Brighter' primaries (CMY) for - spaces.

  • RGB is not uniquely defined!

RGB Color Matching Functions

Cylindical Color Spaces

Newton's and Goethe's color circles

avoid when possible

CIE Chromaticity Diagram

  • $Y \sim$ luminance, $XYZ$ is additive

  • $x=\frac{X}{X+Y+Z}, \; y=\frac{Y}{X+Y+Z}, \;z=1-x-y$.

  • There is no pink in the rainbow!

  • CIE 1931 xyY is (nearly) perceptually uniform.

  • Calibrated for "Standard Observer" (2°, fovea).

  • Standard Daylight: D65 ($T=6504 K$)

Hue vs. Lightness

Lightness and saturation is more important than hue in guiding attention.
But hue helps to indentify and distinguish values.

Adjusting Lightness

For single-hue palettes nonlinear lightness is not problem.

For the multi-hue 'Hot' palette nonlinear lightness is a problem.

Bezier curves in the color space can smoothen lightness.

The lightness can simply be linearized.

How to linearize the rainbow?

Why are there still Categories in the Rainbow?

  • Brightness: how much light a colored area seems to emit.

    Lightness: brightness relative to a reference white (~relative luminance)

  • The color spectrum is not perceptually uniform.

  • Words for colors shapes out perception.

For what Type of Data is Jet even intended?

  • Sequential: ordered hues, linear lightness

  • Diverging: ordered hues with center, (bi)linear lightness

  • Cyclical: cyclical hues, cyclical lightness

  • Qualitative: distinctive but uniform hues, distinct color names, varying lightness

Jet tries to be all, and is none.

A practical Guide to choose good Colors

  1. The colors need to fit the data, and be accessible.

  2. The colors need to fit the application.

  3. The colors should guide the attention and interpretation.

  4. The end product should be pleasant to look at.

1. Meaning - The Colors need to fit the Data, and be Accessible

Data ~ Colors

  • Data Type ~ Palette Type

  • Context ~ Number of Hues

  • Data Values ~ Lightness Values

  • Value Types ~ Continous/Categorical

  • Missing Data Points ~ Extra Color

  • Value Distinctions ~ Color Name Distinctions

Accessibility

  • Color deficiencies (8% in men, 0.5% in women)

  • RGB screens

  • CMYK prints, black/white prints

  • Low contrast beamer

2. Engagement - The Colors need to fit the Application

Color perception is strongly influenced by context.

Look out for influences by

  • background color (dark-is-more, opaque-is-more)

  • amount of 'ink' (alpha, linewidth, bar height, ...)

  • neighbouring and overlapping colors within figure

  • existing color schemes in related figures

Pysiological Basis: The Opponent Process

3. Intuitive Understanding - The Colors should Guide the Attention and Interpretation.

Colors themselves have meaning.

  • Colors can evoke emotions.

  • Colors are associated to concepts.

  • -> highlight, discriminate, guide visual flow

  • depend on context and cultural background

  • Beware of conventions in the field.

one Color - many Concepts

many Colors - one Concept

Mongillo et al. (2018) "Inhibitory connectivity defines the realm of excitatory plasticity"
Nature, doi: 10.1038/s41593-018-0226-x

4. Aestetics - The End Product should be Pleasant to Look at.

Aestetics are directly related to Understanding and Engagement.

  • Aesthetics vs. Discrimination

  • Color Harmony?

  • Color preferences
    ~ preferences of associatiated concepts

  • Taste is subjective.

By Wesley Freyer,( CC BY 2.0)

  • Less is more.
    Use colors sparingly. Use subtle colors.

  • Use color combinations from nature.

  • Copy from your favorite artists.

References & Further Reading