Making figures and slides
for the colorblind
Published April 01 2017
Color coding of objects is a useful and important way to convey information in biochemical studies. Examples include double staining in confocal micrographs and the use of a range of colors to denote variable gene expression. However, one in 12 males and one in 200 females are red-green colorblind. In an audience of 100 men and women, about four people may be colorblind. For effective communication, we must consider colorblind scientists when making figures and PowerPoint slides.
Colorblindness is not the loss of perception of all colors (a condition called monochromacy). Rather, colorblindness makes it difficult for the person to distinguish between certain colors. The primary colors are red, yellow and blue, while the secondary colors are purple, green and orange (Figure 1). Visual color is sensed by three types of retinal cone cells corresponding to the primary colors of red, yellow and blue. Defective red cone cells result in a color blindness called protanopia (from Greek “prot” for the “first” type of cone), while defective green cone cells result in deuteranopia (from Greek “deuter” for the “second” type of cone). Both of these deficiencies are transmitted by X-chromosome-linked inheritance and result in red-green colorblindness. Note in Figure 1 that blue and yellow are perceived identically by normal and red-green colorblind individuals.
For both protanopes and deuteranopes, distinguishing red from green is more difficult than distinguishing yellow from green (Figure 1). Avoid having red and green lines cross or placing red and green objects close to each other (Figure 2). Although yellow has good color character for both the protanope and deuteranope, it may be too close to white for the normal eye to distinguish unless it has a black border or background. Black, white and gray are easily differentiated by nearly everyone.
Of all of the hues, blue is perceived uniformly as a color by all individuals. The color should be used first when generating colored images and slides, followed by yellow or green, depending upon the background. Red is perceived as black by protanopes (Figure 1), so red print over a black background may be invisible to them. Also, the use of dark-red text for emphasis when incorporated within a string of black text will not be perceived differently by protanopes. Moreover, dark-blue text may be perceived as black by everyone, so it may be better to use sky blue or Carolina blue for highlighting. With care, two different shades of blue may be distinguishable. It helps to examine such figures generated on a color printer to ensure that different shades of blue can be differentiated. If it makes no difference whether an object is red or black, it’s all right to use red for the many people for whom red is a favorite color. Be aware, however, that the protanope will see it as black. White, gray and black are not colors, but they are distinguishable and useful.
You can check your colored figures for how the colorblind perceive them by using the freeware at vischeck.com. You can upload your figure and see how it appears to a colorblind person, or you can download the program and make analyses and revisions as necessary. This is the procedure for generating the figures for this piece. For additional advice on preparing color figures, visit here.
A suitably made color figure may be worth more than a thousand words because of the additional information that color coding conveys. However, the coloring should be made so that it can be perceived and differentiated by all scientists, colorblind or not. Figure 1. A color wheel illustrating the colors as seen by (A) a normal individual, (B) a deuteranope or (C) a protanope IMAGES COURTESY OF ROBERT ROSKOSKI JR. Figure 2. A gene-expression profile indicated by red, black and green matrices
Robert Roskoski Jr.
is the scientific director of the Blue Ridge Institute for Medical Research.