Why does the colour of Earths oceans vary?
The color of the ocean water does indeed vary depending on the chemical composition of the water. In addition, the color of the ocean in satellite images is affected by the position of the satellite, angel of the sun, and weather.

Color of water in general

Water appears blue because long wavelengths (red or yellow light) are absorbed more efficiently in water than short (blue or green) wavelengths. Therefore, when a water body — in a swimming pool, for example — is illuminated by a white light source (the sun) the light that scatters back from the water tends to be blue rather than red.

The color of the water can further be altered by its chemical composition. In nature, the apparent color of water also depends on the light source, that is, the weather. The same ocean can appear very different in bright sunlight and on a cloudy, stormy day.

Color depends on substances in the water

The color of the ocean water can be greatly altered by substances dissolved or suspended in the water.
Typical substances that affect the color are chlorophyll (from waterborne algae) that make water appear green, or suspended particulate matter, such as silts and clay, that cause yellow or brown color. These are clearly seen in coastal areas, where local plankton blooms can turn water bright green, or river plumes carrying a high concentration of suspended particulate matter can be clearly identified from their brown waters. Other substances can alter the color of water as well. Colored dissolved organic matter (CDOM) related to decaying detritus, can turn water yellowish or green, for example. Water temperature and salinity themselves do not significantly affect the color of the water.

The color of the ocean, therefore, does depend on the location. For example, waters in areas with high primary production tend to be more green.

Color of oceans in satellite images

In satellite images, the color of the ocean can be influenced by the relative angle of the sun and the satellite versus the ocean surface. The ocean can appear darker if the satellite is at an oblique angle with respect to the ocean surface and/or if the sun is shining from an acute angle. In the case of geostationary satellites, the ocean near the equator can therefore appear to be brighter than in the high latitudes.

Moreover, the color of oceans may be affected by the surface roughness. Surface waves affect the way light is reflected and refracted in the surface and may cause the ocean to appear darker.

Consequently, the color of oceans may vary greatly in satellite images due to the track of the satellite and local weather conditions. This is often visible in large composite images, made out of multiple satellite tracks, where the ocean may appear patchy with different colors.