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We found that users do not always know what is best for themselves. Our design once included groups, which were displayed with user-chosen colors. Users could chose any color from a color swatch - their freedom was enormous! We imagined users carefully picking colors that were easy to distinguish.
Instead, this happened: 
As at least two of our heuristic evaluations commented, some color choices make the name of the company incredibly difficult to read. Our testers weren't employing the careful consideration we'd imagined, and our interface lost usability.
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