Somebody asked me if 青の and 青い 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 in 青の 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 青い空 but you can’t say 青の空.
To describe a natural colour, use an いadjective
I ate a red berry.
I collected yellow leaves
（✖）黄色の葉っぱを集めました。(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) 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 青い and 青の.
The same rule applies when you want to say “xxx is [a colour].”
This strawberry is white.
Wedding dresses are usually white.
Judging from what you have read so far, what do you think 赤いペン means? A pen teachers use to correct students’ mistakes?
- 赤ペン is the most common way of calling a pen with red ink which teachers often use to mark students paper.
- 赤インクのペン is the most precise way of calling a pen with red ink though it is a bit mouthful.
- 赤のペン should mean a pen with red ink but 赤ペン is more common.
- 赤いペン 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.
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 赤くなる vs 赤になる also, which is explained in this post.