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虹は7色だ。
Rainbow has 7 colors.
There are 7 colors in rainbow.
Question:
how to pronounce 7色 to make meaning [7 colors]?
Problem:
七色 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.
虹は七色だ。
[niji-wa nana-iro da]
Rainbow is the seven prismatic colors (thing).
The counter for color is 色（しょく）. If you want to say "seven (different) colors" without any idiomatic meaning, you can say ななしょく.
地図を[7色]{ななしょく}に塗り分ける
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つの色【いろ】.
The word 七色【なないろ】, 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 七色の声.
