Orange is a new word in English. To really get to the concept of orange as youre thinking of it, the description would have been "yellow-red" (ġeolurēad).

Youve opened a huge can of worms with this question and if more people notice it you're going to get a lot of crazy answers that people picked up from listening to semi-scientific podcasts and questionable youtube videos. This is an attempt to hopefully stave some of that off.

Gonna do this in sections.

Basic colour terms

In lingusitics we have what are appropriately called basic colour terms. In English these would be all the words you can think of that primarily mean colours but not something else. Lavender is a flower, it doesnt count. Red means red, and while it also means other stuff like Communist, it's primary meaning is the colour and the other meanings come by association.

Basic colour terms develop in a somewhat predictable way in languages. The initial work on this was Berlin & Kay, published in 1967 if memory serves. This has been revised and tweaked over the years, but the initial description was as follows.

Black and White, or rather Dark and Light, will be the first colour terms that a language will develop. Some languages are still this way.

The next will be red. Then either green or yellow, and then the other one of the two.

If you have black white red and green, the English labels are actually not super helpful here, because actually what this means is dark, light, warm and cool. In a language that has only those four as basic terms, the colour of a standard American school bus falls under "warm". It's not that people think school busses and Coke cans are the same colour. They'd just class both hues under a single category, the way that lavender and Mauve are both classed under "purple" in English.

After green and yellow you'll start to get things like blue, and orange comes later.

You can kinda think of is this way: In English we have pink and pink is pretty basic, but there is no equivalent for blue. We say light blue, sky blue, but there's no single word link pink. That doesn't mean we can't distinguish or describe light blue. We just consider it a shade of blue. In Russian, however, they do have such a word. Nothing has changed in perception, just classification.

A number of the languages I work on are in the 4-5 term stage. Some don't have a word that we could neatly translate for "yellow", while others have one, and now most of the varieties in the area are working out what to call things that you'd call words like blue, grey, purple in English. They can still discuss all these colours, and in other ways their colour-description system is much more complex than what we have in English, but they just don't have terms that would qualify as "basic" according to Berlin & Kay's system.

I mentioned this has been tweaked over the years. This is mostly in the context of realising not every language follows the order as neatly, and some things like grey can be something of a wildcard. The core idea still stands.

Colours don't real

What I mean by this is that when you think of "red", you have an idea of what that is in your head. If I gave you a bunch of red things, you'd probably be able to point to one of them as the ideal red. But this ideal is entirely culturally conditioned. There is no point where red objectively turns into orange if we're going through a colour wheel. That we divide a rainbow into the colours that we do is not based on anything objective or scientific. It's all culture. And culture changes. So what you now may think of as the actual whatever colour terms and their referents, you great great grand kids may think you were a bit slow for not recognising their "objective" reality.

Red hair has been called red throughout the world, but then so has blonde hair. The Dutch in early Chinese language sources relating to their interaction with Taiwan refers to them ang-mo 紅毛, "red hairs". Traditionally, Sinitic had a word for black 玄, for white 白, for yellow 黃 (and red 朱 and green 青), but a form of "red" was the term they went with.

Orange hair

Back to English. Orange is now a basic colour in English, but the word comes from the fruit. But before the 16th century, it wasn't a thing. That's still a long time that we could have started calling red hair "orange hair", but there's one more thing that comes in to play: We don't say "yellow hair" for blondes. The words we use to describe some things, in this case hair, aren't always the words we use to describe other things of the same basic quality. Hair is not described with the same words as other things. Eyes as well.

As a sort of analogy, in one of the languages I work on, they have two words we could translate as green. But one of them is only ever used to describe the colour of plants. You can have a green piece of paper and it's called one colour, but then if you cut that paper into the shape of a leaf, it is now the other colour. It doesn't matter if it's a real leaf or a representation of a leaf; it's still the plants-only "green" that it gets called. That's just the convention.

While this isn't the norm for all languages and all colour terms, it's a common enough occurrence that it's not surprising when encountered.

Conclusion

So yes we could totally say orange hair, and I'm sure kids surely do, but it's not the convention, and when those kids say it, they're probably corrected by their parents or that know-it-all Timmy in their class.

I hope that helps answer your question. It ultimately boils down to convention, but a convention based on a somewhat complex system of lexical development. Let me know if you have follow ups.