Peole use both 蓝天 and 青天 and I am wondering if there is any difference. While 蓝 means blue, 青 means cyan, right?
Why do people say 蓝天白云 often but seldom 青天白云 (though 青天白日 is sometimes used)?
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蓝天白云 and 青天白日 are idioms, so they cannot be changed.
蓝色 means blue, but 青色 is arguable.
In the colour printing technology, the jargon term 青色 is defined to be cyan. However, if you are not talking about the colour printing technology, 青色 can mean blue, cyan, green, or even black.
青天白日 blue sky and white sun
绿水青山 green water and green mountains
山青水秀 the mountain is in green, and the water is beautiful
青，取之于蓝，而青于蓝。 Cyan, is made from blue, but cyaner than blue.
雪青色 violet colour (lit. bluish snow colour)
青草 green grass
青衣 black clothes
青布 black cloth
When does 青 mean blue? When does 青 mean green? When does 青 mean black? There are no rules and no logic for it, just idiomatic. For example, many people use 青草 for “green grass” and therefore 青草 becomes the idiom of “green grass”.