I know this question has been asked before but I remain confused about the use of nouns or i-adjectives in predicate position for colours, i.e. 海は青です versus 海は青いです.
I have received the following advice in comments to a post:
More importantly though, 青くない and 青ではない are not interchangable. 海は青ではない kinda sounds to me like 海 is being used as the name/label of some color, which happens to be different from blue.
The thing is, 青 does have an explicit adjective-い form, so when the speaker nonetheless chooses to use the non-adjective form, I tend to expect a grammatical reason for that choice of words (in the absence of context at least). I guess there could be other stylistic reasons to prefer the non-い-adjective form that I'm not aware of; but at least, I think it deserves mention that 青ではない is less common/usual/natural than 青くない in most scenarios.
I cannot find consistency in the answers in this site as to what the truth is here. If I look at this question, for example, one answer seems to back up the above quotes and the other does not. In this question the answer claims that the noun form should be used if available, which contradicts the above.
I also see sites on learning Japanese (which, granted, are not necessarily a reliable source of natural Japanese) that show basic 'X is colour' sentences using the noun form e.g. this site.
This all leaves me very confused. Is there a distinction at all? Is there a distinction, but one that mainly only exercises purists? If there is a distinction could someone please elaborate on when it is correct to use each form?
A final thought: perhaps the fact that this sentence is in negation affects the most natural choice. Maybe 海は青です is natural but 海は青ではない is weird. Could someone comment on this?