Almost all conceptual images are generated digitally and in full-colour. This allows colour to be used to distinguish, or relate, visual elements. However, while colour images are used online, these images are often printed in black and white for hard copy journals and textbooks. Some thought must therefore be given to how that colour image will work when converted to greyscale. Will the colour-coding of elements still work or will the conversion to greyscale render that meaningless, making the figure more difficult, or impossible, to read?
The image below, left was re-drawn from an existing conceptual image. Colours are used to effectively distinguish between four different groups of neurons in the olfactory region (assuming the viewer is not red-green colour blind). However, if this colour image is automatically converted to greyscale (using Adobe Photoshop) the resulting image is shown below, centre. Here, there is no longer a clear distinction between the four groups of neurons. Instead, it appears as if there are just two groups of neurons, one dark grey and one light grey. By contrast, the image below, right has been specifically drawn in greyscale, with the grey tones chosen to better distinguish between the groups of neurons. The background has been simplified slightly, with no colour gradients and thinner black outlines, to allow the main elements to stand out more clearly. By necessity, figures rendered in greyscale can contain less subtleties of tone than colour images. This can make them less successful at visually communicating complex figures, such as this example.
Typically, a colour image is supplied to a journal and it is the journal that will generate the greyscale image automatically. Therefore, it makes sense to check that your colour image will still work in greyscale.
The image below, left was re-drawn from an existing figure and uses bright colours to distinguish between three nuclei. However, if this image is converted automatically to greyscale it results in the image shown below, centre left. The three nuclei are no longer so clearly distinguishable and the previously green nuclei is now too pale to stand out adequately against the white background. By making relatively small adjustments to the colour image (shown below, centre right) to accentuate the outlines of the nuclei and to increase the differences in tone, the resulting greyscale image (shown below, right) is much easier to read.
Anatomical images can work well in greyscale, although it may be better to simplify the image slightly and avoid too many subtleties in tone and gradients. The greyscale image of the brain below, centre has been automatically generated from the colour image to the left. The greyscale image below, right was drawn specifically in greyscale, and contains no colour gradients and slightly less detail. Consequently, it probably works better as an image in its own right.
If you know that your colour figure will also be printed in black and white, it is worth spending some time checking how your image will appear in greyscale and, if necessary, making small adjustments to the original colour image. Ensuring that different visual elements vary in tone, as well as in colour, or adding outlines to elements to make them stand out, may help when the image is converted to greyscale. Keeping the figure as simple as possible, without unnecessary colour gradients or effects, will also result in a clearer greyscale image.
If your figure will only be reproduced in black and white, then draw it in greyscale from the start, as this will ensure that it will visually communicate your work effectively.