Zebra because it is part of the Zebra Mbuna complex, which includes blue with black zebra bars, solid blue, solid white, solid orange, orange blotched, OB white, and even mustard yellow plus OB yellow.
Why "red"? Different opinions. There is a tradition with Koi calling deep orange "red", thou not sure if that is the reason. Some claim that the '"Red Zebra" look reddish to the human eye in the native lake, orange-red wavelengths are absorbed faster in water. The "Red Zebra" certainly are a deeper orange than other Mbuna. Male "Red Zebra" are usually not orange in the lake but light blue, which adds to the confusion.
IME lots of fish that have orange or even yellow on their bodies have those features called red. I always thought it was marketing. Red is rare so it is in demand. Like some flowers are called blue that are purple in person. Except maybe as noki says, in certain light.
Thanks, just had to ask, orange fish without stripes being a red zebra doesn't make sense. Lol
I do know that they look red in the aquarium at night with blue lighting.
Thanks for answering my dumb question.