Colors for the Sake of Color
I wouldn’t be surprised there’re fMRI studies showing more brain processing when people look at more colorful scenes, but that doesn’t mean it’s bad. I don’t think there is a simple direct relation between number of colors on the screen and cognitive stress, let alone a threshold number of colors you want on the screen. Photographs and photo-realistic images, for example, may have a huge number of different RGB values spanning the visual spectrum, but I don’t think they make web pages “harder” to process. Indeed, sometimes you need 24-bit full-color photos to make the details easily recognizable.
As long as the things that users need to see contrast well with their surroundings (e.g., words versus their background), I don’t believe the colors on the screen impact user performance much. I remember back when color monitors were new that there were some studies (I’m afraid I lost the cites) comparing color versus monochrome monitors, and they generally found no difference on things like task productivity (granted these early monitors could only show a few colors at a time, so maybe it wasn’t a real test of what you’re asking about).
In most contexts, the UX of color is primarily aesthetic: if it looks good, then users will feel good. As a general rule, too many colors on a web page can look amateurish and busy, and doesn’t give the page a cohesive mood, but it’s more art than science.
Things change when you start talking about color coding –that is, using color to represent a data value or meaning in a graphic display. There have been decades of research on that, and entire manuals written. This research has shown that user can visually distinguish 100s of 1000s of colors but only if the colors abut each other on the screen, and the ambient light is relatively subdued.
In contrast, if users need to identify the meaning represented by color shown in isolation on the monitor, then the number of colors users can accurately process is dramatically less –about 6 colors at most, and that assumes you’re using the full spectrum (e.g., using something like red, amber, green, blue, magenta, and white). Also, except for some cultural conventions (e.g., red for danger, hot, and stop in western countries), color codes tend to be arbitrary, and asking users to memorize more than 6 arbitrary things is taxing.
The number of colors for codes is also limited by the constraints on the choice of the exact color values you should use:
The apparent color of colors changes with the color of the context –including the color of things they’re next to, so colors need to be very distinct from each other to avoid confusion.
Colors need to be not only distinct from each other but distinct from the background they may lie on. If you have a multi-colored background, that can be difficult.
If the display is used in bright ambient light (e.g., a smart phone used in sunlight), your colors will likely need maximum saturation to be distinguishable.
Colors should be consistent with cultural expectations (e.g., don’t use green to represent an alarm condition).
Red-green color blindness is relatively common, so colors should not be distinguished solely on redness and greenness (full accessibility requires some non-color redundant indication be provided for the small minority of users that are completely color blind).
Bright high saturation blue makes objects appear a bit blurry, especially for older users, so avoid bright blue when users also have to see details (e.g., for small text).
For documentation and phone/text technical support, colors should correspond to common color names. You don’t want to have to say, “Look for an umber icon.”
If you’re thinking these are frequently contradicting guidelines, you’re right. The net result is sometimes you have to have fewer than six codes. Balancing all these guidelines for a given combination of users, tasks, environment, and technology takes research and design judgment.
I’ve more details about color coding at Breaking the Color Code. For a set of research-based guidelines on color coding, see my colleagues’ paper for color on air traffic controller displays.