* Historical Naming: Historically, the vocabulary for describing colors was less precise. Many languages had fewer distinct terms for colors than we do today. In the past, what we now call "orange" was often considered a shade of "red."
* Dominance of Reddish Pigment: Genetically, "red" hair (what we'd call orange) is caused by a mutation in the MC1R gene, which results in a higher concentration of the pigment pheomelanin. Pheomelanin produces a range of colors from strawberry blonde to deep auburn. But the red component is the most noticeable and defining characteristic, hence the umbrella term "red hair."
* Cultural Convention: Once a term is established, it tends to stick, even if it's not perfectly accurate. Calling it "red hair" is simply the convention that evolved and became widely accepted.
* Varied Shades: Red hair encompasses a wide spectrum of shades, from strawberry blonde to copper to auburn. While some of these lean more orange, others are definitely redder. Using "red hair" as the umbrella term covers all these variations.
* Simplicity: "Red hair" is just a simpler and more concise term than trying to describe the specific shade of orange each time.
In essence, "red hair" is a historical and cultural artifact. It's a legacy of a time when color vocabulary was less refined, and it has stuck around as a convenient shorthand for a range of reddish-orange hair colors.