The answer to 'why is my gutter discolored' is that gutter discoloration is often caused by a buildup of dirt, debris, and algae over time. This can result in a grimy, stained appearance in the gutters.
Some common causes of gutter discoloration include leaves, twigs, and other organic matter that accumulate in the gutters, as well as mineral deposits from hard water. Algae growth can also contribute to a green or black discoloration. In some cases, the material the gutters are made of (such as aluminum) can oxidize and discolor over time as well.