The pigment responsible for the cyan-blue color of plants on a planet around a white dwarf star may not necessarily be phycocyanin. Phycocyanin is a pigment found in certain types of cyanobacteria (blue-green algae) and some marine plants and algae. White dwarf stars emit a different type of radiation compared to main sequence stars like our Sun. Depending on the composition of the planet's atmosphere and the specific wavelengths of radiation it receives, the color of plants on such a planet may appear differently than on Earth. Different pigments, such as chlorophylls and carotenoids, contribute to the colors we typically associate with plants on Earth. However, the specific pigment responsible for a cyan-blue color in the plants on a hypothetical planet around a white dwarf star would depend on several factors, including the specific evolutionary adaptations of these plants to their unique environment. Without further hypothetical specifics or a known example, it is difficult to determine the exact pigment that would cause cyan-blue coloration in plants on such a planet. The color of these hypothetical plants would likely be a result of a combination of various pigments that enable them to absorb and utilize the available light efficiently.