Help with colors in Japanese?

Question:

I'm a bit confused with how to use the noun and adjective forms of colors in Japanese.

I know that for the majority of the colors, adding 'i' at the end will make it an adjective, while leaving the 'i' out will make it a noun.

For example:

Kono hana wa akai desu.
[This flower is red.]

Akai hana.
[Red flower.]

Aka wa kirei desu.
[Red is beautiful.]

However, how come the manga, "Ao no Exorcist" is written like that? That confuses me :(
I thought that 'no' was only used for colors like purple and green?

Answer:

To get inspired, I seached "Ao no Exorcist"
You want ”青の祓魔師（エクソシスト）” to be "Aoi Exorcist", right?
"aoi exorcist" sounds like "Blue(-colored) Exorcist", but actually 青 here seems to describe "a blue flame" that Okumura Rin gives off. 　”青の（炎を発する／放つ）祓魔師”

Note: "Titles" are tricky. They sometimes ignore basic grammer, the same way as you see in English titles/headlines, which might explain what area52 said.