In this task, we ask you to elaborate the sentence without changing its general meaning. You can do so by explaining further the input sentence, using more precise wording, adding qualifiers and auxiliary information etc.
Q: Algal blooms are often green , but they can also be other colors such as yellow-brown or red , depending on the species of algae .
A: 
The photosynthetic pigments in the algal cells determine the color of the algal bloom , and are thus often a greenish color , but they can also be a wide variety of other colors such as yellow , brown or red , depending on the species of algae and the type of pigments contained therein .