This experiment shows that the selection is based on phenotype frequency, not on phenotype itself (which would be directional selection). Fish do not prefer one particular color, just the color that is most prevalent at given place or time.
Work Step by Step
Consider the two kinds of selection: directional selection acts for one phenotype and against another, consistently. Frequency-dependent selection varies the selected phenotype by how common it is. Clearly the results here fit the latter case.