An Algorithm for Generating Color Scales for Both Categorical and Ordinal Coding

Previous research has shown multihue scales to be well-suited to code categorical features and shown lightness scales to be well-suited to code ordinal quantities. We introduce an algorithm, Motley, that produces color scales varying in both hue and lightness, intended to be effective for both categorical and ordinal coding, allowing users to determine both absolute and relative quantities efficiently and accurately. The algorithm first determines the lightnesses of scale colors to maximize perceived lightness differences and establish the lightness ordering, generating separate search spaces for each scale position. It then selects hues by heuristic search to maximize the discriminability of the scale. It produces scales that are ordered with respect to lightness but unordered with respect to hue and thus more discriminable than typical multihue lightness scales. In an experimental evaluation on human subjects, Motley’s scales enabled accurate judgments of relative quantity, with response times superior to unordered multihue scales and comparable to ordered lightness scales, and enabled accuracy and speed of judgments of absolute quantity superior to lightness scales and comparable to multihue scales. yPublished 2009 Wiley Periodicals, Inc. Col Res Appl, 35, 18 – 28, 2010; Published online 17 November 2009 in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/col.20559