It sounds like the brown-orange link is intuitively obvious to you. But I think the claim being made is that for most native English-speakers, the link between orange and brown is significantly less obvious than that between X and dark X, for most colours X. FWIW that is true in my case.
(Not that I deny the link exists; but when I look at e.g. an orange next to a dark brown tree branch, I don't see them as versions of the same colour in the way I do with, say, a lime and a dark green leaf. That's not a great example but hopefully you see what I'm getting at.)
(Not that I deny the link exists; but when I look at e.g. an orange next to a dark brown tree branch, I don't see them as versions of the same colour in the way I do with, say, a lime and a dark green leaf. That's not a great example but hopefully you see what I'm getting at.)