How to use colours effectively in scientific figures?

Often enough I see schematic representations in scientific papers or presentations that look something like this:

ColourFigureExample — An example of a scientific figure using colours randomly.

As many as seven colours are used, with same level of intensity (red, blue, yellow, orange, violet, green, magenta). The outcome looks… colourful. The downside is that this colourfulness does not help carry your scientific message. At this level of colour intensity and abundance, there is no hierarchy and no more distinction between elements. This image could be in black and white instead and the level of information would be exactly the same.

Here are some ideas how to use colour more effectively:

choose maximally 2-3 colours per figure and vary their shades and saturation to increase the number of colour combinations;
use complementary and / or neighbouring colours for colour harmony (colours opposite each other or neighbouring on the colour wheel). For example: orange and blue are a complementary pair, magenta and violet are a neighbouring pair;
use warm colours (colours closer to red on the colour wheel) to push elements visually into the foreground. Analogously, cold colours will push them visually into the background so use them for elements that play a secondary role;
create contrasts by introducing pale and intense shades of the same colour instead of only using different contrasting colours of similar intensity;
be consistent in your colour code: if you have several figures in a presentation / publication highlight the same elements with the same colours in all of them (varying shades and intensity if necessary). This way the colour code helps your audience read the figures more efficiently;
use spot colour: highlight in colour only the elements that really constitute the core of your message. Keep the rest of your image in grey scale. This is a very powerful way of using colours.

Same figure applying some of the guidelines mentioned above:

Colour Scientific Figure Restricted Number of Colours — The figures use three colours: red, orange and blue. Additionally shades of red, blue and orange are employed. Both figures highlight in particular the elements in intense red, orange and blue with secondary elements being paler and appearing to be in the background. However the figure on the left seems to focus mainly on the two receptors connected via a common ligand due to highlighting in red and orange. The figure on the right also draws attention to the outcome of this connection and the reaction in the cell nucleus.

Colour scientific Figure using Spot Color — This figure uses spot colours. Colouring only some elements and keeping the rest in grey scale can actually be the most effective way of highlighting chosen elements in figures.

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