The most likely culprit is rust. Your water heater has a metal tank, and over time, the inside can start to corrode.
Think of it as a very slow, underwater version of a old car fender. That red-brown tint is just tiny iron particles saying hello.
Fun fact: A single tea bag can turn a whole pot brown. Your water heater is like a giant, rusty tea bag—but for iron flavor, not flavor.
