７色 vs 七色 for rainbow (7 colors vs 7 prismatic colors) > 7 > Rainbow has 7 colors. > There are 7 colors in rainbow. Question: how to pronounce 7 to make meaning [7 colors]? Problem: is not [7 colors]. It is a special scientific term, meaning [7-prismatic-colors]. We do not make [7 colors] meaning, if we pronounce this sentence like this. > > [niji-wa nana-iro da] > Rainbow is the seven prismatic colors (thing).

The counter for color is **** . If you want to say "seven (different) colors" without any idiomatic meaning, you can say .

> [7]{}
> to paint a map using seven different colors

When you say like this, the colors can be anything including gray, brown and pink. Alternatively, you can just say 7.

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The word , in kanji, specifically refers to the seven colors of a rainbow. (I wouldn't call this a scientific term...) It also means "a wide variety of", as in .