When multiple groups are shown in a scatterplot each stratum is represented by a different symbol; for example, three strata might be coded using red, green, and yellow circles. Various symbol types were compared by behavioral experiment: Subjects were fastest when strata were coded using different colors and slowest when strata were coded with confusable letters—but there were no differences in accuracy. Accuracy differed only when processing time was restricted, again with different colors and confusable letters representing the two extremes. We conclude that color is the optimal symbol type and show that measuring response latency in addition to accuracy is essential in research on graphical perception.