青い海 vs. 青信号、青空 - when can you drop the い?

Hello!
Been scrolling through a graded reader and was confronted with something that has confused me for a while. Is there a rule or explanation for when you add the い to the i-adjective colors and when you dont?
Are these just fixed compound words, as in theres a limited amount of them you could learn?

In my search i got that the color, in e.g. 青空 act as a compound noun but how does that work? can any i-adjective do that in certain situations? (and na-adjectives?)

if we look at e.g. 青りんご - would i have to use it differently if i said “Green apples are healthy” or if one green and one red apple were lying in front of me and i wanted to say “i want the green apple”
→ Is it just a way of making it a broader term? to show youre talking about them in general and not a specific one?

Thanks!

Essentially, yes. Things like 青信号 and 青空 are existing words that just happen to have colors in them. You basically have to remember them as you encounter them. You shouldn’t try to create one like that by “removing” the い yourself.

※At the same time, it’s worth noting that 青 on its own is a noun (in contrast with the adjective 青い), and so there may be times when using the noun is appropriate. For instance, if you were to say “I like blue,” that would be 青が好き. Just wanting to be clear.

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