How to choose adjective or noun with color

Question

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.

and

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?

Answer 1

白い, 黒い, 赤い, 青い, 黄色い and 茶色い are the only Japanese i-adjectives for colors, as probably any textbook says. For those colors, you should be using those i-adjectives in more than 90% of the time. However, there are cases you can (or should) treat 白/黒/赤/青/黄色/茶色 as nouns (aka no-adjectives).

• The largest reason to say 青の instead of 青い is when you want to use it as a symbol/label rather than just a color. For example, "Red Team" is either 赤チーム or 赤のチーム but not 赤いチーム. When I hear 青の財布 without any context, I wonder if it's really a blue wallet; it may be a non-blue wallet with a blue tag attached or contained in a blue box. You can hear 赤のお皿 in sushi-go-round restaurants because the color of a plate indicates the price of sushi on it (saying 赤いお皿 is also correct, though). You can modify an abstract thing using の, e.g., 赤の在庫 ("the stock of the red ones"), 黒の注文 ("the order of the black version"), 青のイメージ ("the image of blue(ness)"). See this question: の (no) vs ~i for colors, can they be used indistinctly?
• Another common reason is to keep your expressions short and consistent when you want to describe the colors of many objects. For example, it's natural to drop all い and say "その本は青で、そこのボールは緑で、あそこの屋根は赤です". Instead of saying 茶色く/黒く, you can say "髪は茶色で、服はピンクで、瞳は黒で塗ってください".

So 海は青だ is okay if this is said in a context involving many colors. But 海は青だ said without any context sounds odd, and someone who reads it would (unconsciously) wonder if it has some "symbolic" meaning in the context. If there is no other context (e.g., if this is a catchphrase on a poster), it may look like "Blueness is (the symbol of) the ocean".