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In 1959, Hanson did a similar experiment. But in addition to training the pigeons to peck at the S+ color, he also trained some to avoid a similar color, called S-. In one case the S- color was very similar to the S+ color, and in another, the S- was more distant from S+.

When they were presented with a range of colors, the "control" pigeons (trained only to peck at S+) showed a pattern of generalization -- just as in the Guttman & Kalish experiment.

However, in the pigeons trained to avoid S-, the peak of the generalization curve was shifted away from the S- color.

     
     Redrawn from Hanson (1959) (full reference)

In the case of the S- color that was distant from S+, the peak shift was small. But when S- was very close to S+ (555nm and 550nm, respectively), the peak shift was stronger.

Does this look a bit familiar? (hint: utility function)