青い vs 青の

Somebody asked me if 青あおの and 青あおい are interchangeable. Both can be used in front of a noun and mean “blue”, so in many cases you can swap them but not all the time.

青あお in 青あおの is a noun. Nouns for colours can be used to describe one of the alternatives. This boy could have worn any colour shorts, so here 青あおの can be used. However, sky is naturally blue and it doesn’t change its colour at will. Then, you need to use an “adjective” to describe it. That’s why you can say 青あおい空そら but you can’t say 青あおの空そら.

To describe a natural colour, use an いadjective

I ate a red berry.
（✓）赤あかい実みを食たべました。
（✖）赤あかの実みを食たべました。

I collected yellow leaves
（✓）黄色きいろい葉はっぱを集あつめました。
（✖）黄色きいろの葉はっぱを集あつめました。(This sentence is OK if the leaves are artificially painted yellow.)

To describe something artificial, usually either is fine

If you bought a pair of blue shoes, you can say both:

(a) 青あおい靴くつを買かいました。
(b) 青あおの靴くつを買かいました。

(a) is just stating you bought a pair of blue shoes.
(b) is implying you could have chosen another colour but you decided on a blue pair.
However the difference between the two is very marginal and you can say you can interchange 青あおい and 青あおの.

The same rule applies when you want to say “xxx is [a colour].”

This strawberry is white.
（✓）このイチゴは白しろい。
（✖）このイチゴは白しろだ。

Wedding dresses are usually white.
（✓）ウェディングドレスはたいてい白しろいです。
（✓）ウェディングドレスはたいてい白しろです。

赤あかいペン

Judging from what you have read so far, what do you think 赤あかいペン means? A pen teachers use to correct students’ mistakes?

• 赤あかペン is the most common way of calling a pen with red ink which teachers often use to mark students paper.
• 赤あかインクのペン is the most precise way of calling a pen with red ink though it is a bit mouthful.
• 赤あかのペン should mean a pen with red ink but 赤あかペン is more common.
• 赤あかいペン usually indicates a pen whose out colour is red. The ink could be red but could be something else.

There are only 6 いadjectives that describe colours.

blue	青あおい
red	赤あかい
white	白しろい
black	黒くろい
yellow	黄色きいろい
brown	茶色ちゃいろい

With these 6 colours, you need to be careful if you are talking about something natural or something artificial/mechanical. The same rule applies when we talk about colour changes like 赤あかくなる vs 赤あかになる also, which is explained in this post.