Rainbow has 7 colors.
There are 7 colors in rainbow.
how to pronounce 7色 to make meaning [7 colors]?
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.
[niji-wa nana-iro da]
Rainbow is the seven prismatic colors (thing).