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 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, being blue is one of the natural characteristics of the sky. Then, an “adjective” has to be used to describe it. That’s why you can say 青い空（あおいそら） but you can’t say 青の空（あおのそら）.
If you bought red shoes, you can say both:
(a) 赤（あか）い靴（くつ）を買（か）いました。Akai kutsu wo kaimashita.
(b) 赤（あか）の靴（くつ）を買（か）いました。Akano kutsu wo kaimashita.
(a) is just stating you bought red shoes.
(b) is implying you could have chosen another colour but you decided on a red pair. However, the difference is very marginal, so in this case you can say you can interchange 赤い and 赤の.
There used to be only 4 colour adjectives in Japanese, which are 青（あお/ao）い – blue、赤（あか/aka）い – red、白（しろ/shiro）い – white and 黒（くろ/kuro）い – black. However, 黄（き/ki）- yellow and 茶（ちゃ/cha）-brown are usually used with the kanji 色（いろ/iro）- colour, so you can use 黄色（きいろ）い/kiiroi – yellow and 茶色（ちゃいろ）い/chairoi – brown as an adjective as well.