The reason 'red' is acceptable to describe the horse, despite the fact that it is clearly not red, is because the concept of orange only entered the English languange relatively recently. Before that it was described as red, and some vestiges of this remain (e.g. redhead).
Imagine a continuous colour wheel. Most people from the same culture today will draw the same boundaries between colors. Red, purple, blue, green, etc. What is to say these boundaries are objective? If in our own language red through orange was once referenced with a single word, why shouldn't red through blue have a single name in ancient Greek?
Red through blue covers a vast segment of your continuous colour wheel analogy, perhaps some two thirds. How likely is it that an ancient culture would have a single word for such a vast expanse of the natural world? Not impossible, just highly unlikely.
Imagine a continuous colour wheel. Most people from the same culture today will draw the same boundaries between colors. Red, purple, blue, green, etc. What is to say these boundaries are objective? If in our own language red through orange was once referenced with a single word, why shouldn't red through blue have a single name in ancient Greek?