The easiest way to tell is to ask about colors in the rainbow. Obviously, rainbow is actually a spectrum with no clear boundaries, so where the boundaries are placed is entirely down to the culture of the person describing it.

In Russian, sky blue is a distinct rainbow color from blue, but pink is not.

In English, blue and indigo are treated as distinct colors, but this is more tradition than perception (so as to pad the number to the requisite 7).

If you compare the typical representation of the rainbow, you'll see that it has the corresponding difference. Russian depiction of rainbow has pure medium-dark blue between sky blue and violet, and violet is dark:

http://nobacks.com/wp-content/uploads/2014/11/Rainbow-26.png

(Note that blue in this pic is about as dark as it gets; quite often it's actually lighter - usually, sky blue and blue are closer together than blue and violet.)

English depiction of rainbow has some kind of really dark blue with a strong hint of violet in the same place, and violet is brighter:

http://weknowyourdreams.com/images/rainbow/rainbow-02.jpg