I'd wager that if researchers asked English speakers: "In one word, how is the right swatch different from the left swatch above?" a common answer would be"Oranger."
Yet, while "redder," "bluer," "greener," "whiter," even "purpler" appear in common dictionaries, the word "oranger" does not. Strictly speaking, "oranger" isn't a word. Still, no native speaker would be confused. Only a few prescriptivist pedants would even bristle.
I was going to explore this a bit, but I've caught a flu and am not really up to it though it is interesting. It would require me to read some linguistics, psycholinguistics, color-categorization theory, and more. Time-consuming and boring, even for me and even if I were not ill, whine whinge whine.
And after all that I still wouldn't be able to say for sure why it was the case. But there are a few things I knew already and I'll type those up.
Of the six main color groups (the primary colors: red, blue, yellow; and the compound colors: orange, purple, green), "orange" was accepted into English usage most recently--at least 300 years after the others (and even then as the name of the fruit, not the color). It had the least "normal" origination for Middle English words -- it's from Persian, not Old English, Old French, Norse, or Germanic or classical roots.
While the citrus fruit was called "orange" in English around 1300, I think it wasn't until 1500 or so that the color was assigned that English name. Before that, the color was "geoluhread," which roughly means "yellow-red."
Thus, the color designation "orange" is more than 500 years newer to English than any of the other basic color words. Unusually for such a common color name, it's newer than the Magna Carta and roughly contemporaneous with the discovery of the Americas.
Some other peculiarities about "orange" -- the color and the word.
- The earliest carrots were apparently not orange, but probably purple or yellow -- 17th century Dutch farmers bred for the orange color.
- In the development of color classification systems in various languages, orange is one of the last major named color concepts to arise.
- That orange rhyme thing, which Tom Lehrer had a bash at.
- The House of Orange, the Dutch royal line, has an etymology apparently unrelated to the English word.
- Paul Putner played Curious Orange and has one of the most ear-piercing screeches ever emitted by a man.