The use of aquatic dyes or pond and lake water colorants is an interesting concept that has some value for helping prevent algae and to reduce the proliferation of certain unwanted aquatic plants and slime and algae. These products work on many levels, but the principal benefit of using dyes is to block light from the sun, llight that can contribute to the growth of algae; also, by blocking the sunlight we can reduce the sun rays that hit the bottom of the pond, as the water is more opaque, thus we are reducing the warming effects of the sun compared to a untreated pond.
Of course, pond dyes are also used purely for their aesthetic quality. Turning your pond blue or darker when green waters are creating a major case of ugliness can quickly solve many problems!