@Alex That's just my personal theory as to why it's that color. It isn't scientifically proven. Though the color red is often a "warning" color in nature and some non-venomous snakes mimic venomous ones, neither is true in this case. Other plausible theories are:
1) It's to scare predators just by looking weird and alarming.
2) It's to attract mates and help them recognize their own species, especially in murky fresh water, red and black might be easy to see.