When is color worse than B&W?

Generally, color displays are better than black and white ones (or monochrome displays, depending on your display technology). In addition to making products look more sophisticated, color lets us communicate more information for a given display size. There's nothing like red to tell you there is a problem and green to tell you things are OK.

Unless you're red/green colorblind.

About 10% of males can have problems with red/green colorblindness, varying with the population you are considering. Most colorblind people don't just see gray, but it is very common for them to have problems distinguishing particular hues and intensities of red from corresponding greens.

If you are designing a product that uses red and green to display important information, then make sure there is a secondary way to obtain that information that works even if you can't tell the colors apart. Some example strategies include:

• Positional information. Traffic lights are OK because the red light is always on top, so you know what color it is by its position.
• Use color only as auxiliary information. If the display is the red text "FAIL" vs. green text "OK" then colorblind folks will do just fine.
• Blinking rates. If you have a bicolor LED, then consider flashing for red and solid for green (which may be a good idea anyway, since flashing lights attract attention). Or a distinctly different blinking rate.
• Significantly different luminosity or brightness. A very dark red vs. a bright green may work out OK, but you should do some testing or dig deeper to be sure you got it right.

Fortunately for me, I'm not colorblind. (This also means I'm not a personal expert on what tricks might work.) But enough people are that this is the sort of thing you don't want to miss when you are making an embedded system. Chapter 15 of my book discusses user interface design and user demographics in more detail.
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