Why are "red tides" red? why are these tides seen at some times and not others? what algal division would be most likely to produce a "green tide"?

The red tides are caused because of the algal blooms, which discolor the ocean's water. These tides are red because of the presence of the pigments known as phycobilin. The blue phycocynin and red phycoerythrin contributes to the red color of these tides. The chlorophyll a contained in most of these algae are seen only when the algal blooms although at other times they are still present but deep in the ocean where they cannot be seen.

The green tides will be produced when the green algae, chlorophyta, blooms. These algae obtain energy from the sun for photosynthesis.

Red tide is a phenomenon which appears when there are large concentrations (a rapid increase of algae population) of unicellular algae. This phenomenon is also known as an algae bloom and it depends on a lot of factors like wind, temperature, nutrients, and salinity. It is usually caused by species of dinoflagellates. Because algae contain photosynthetic pigments that vary in color from brown to red, the water appears to be discolored. The green blooms (“green tide”) in water systems are frequently a result of cyanobacteria.