赤い花 is “a red flower” on the noun detail page, but
赤ペン is “a red pen” on the adjective detail page.
What context am I missing here? I get that “descriptive nouns” act as adjectives (at least in English; I’m level 5 please go easy on me), but these two fragments look exactly the same to me.
If this has already been answered somewhere else, please let me know! (I saw this post, but it doesn’t explain my exact situation.)
The usage of 赤ペン is probably a bit misleading. 赤ペン is more like one compound word.
A better usage of the noun 赤 as a noun on its own would be something like 好きな色は赤です.
In that situation 赤い couldn’t be used, because the matching element of 色 needs to be a noun.
Ok, if I understand you correctly, then some things like (from the detail pages) red pens, red tape, red peppers, are considered compound nouns, whereas red flowers, red jewels, red threads are not? Is there something materially/culturally different between these objects that requires them to be compound nouns rather than adjective+noun?
I think it’s just something you have to keep an eye out for and get used to.
There’s nothing fundamentally wrong about saying 赤いペン, so it won’t cause problems if you use it.
As an aside, if you said 赤花 (あかばな, Epilobium pyrricholophum) that is actually a specific type of plant, so it is much more targeted than 赤い花, which is any red flower.
Some common words where 赤 got combined include:
赤信号 - red light (on a traffic signal)
赤字 - deficit, being “in the red”
Wow, thanks for the details! I didn’t realize that was a particular flower (I just found it in the examples), so that’s helpful. I appreciate the detailed reply!
I think the 赤 in 赤ペン is like the blue in bluebird. It does not mean a general bird that is blue, but a specific bird that just happens to have blue in the name. In the same way that you could say blue bird instead to mean a generic blue bird, you could say 赤いぺん to mean a generic red pen. Why some compound words have a colour as part of the word, while others use an adjective is something that happens over time and differs per language, making it seem mostly random from an outside view.
Not unlike 赤坂. A place name that happens to have 赤 in the name. Or 青山 if we want to stick with blue as the example 
Gotcha! That makes sense. I like your example. I think I got confused by the other examples (esp since I didn’t realize the red flower was a specific flower).
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