Imagine a green light source is at the center of a transparent material sphere and I am staying in the vacuum (or air) and looking to it.
Now imagine that the wave length of the this light increases drastically when it goes out of the sphere.
And I would see the source same color whether I am inside the glass or outside the glass. Although wavelengths change while traveling from one medium to another, colors do not, since colors are associated with frequency.
But why are the colors associated with their wavelengths in vacuum instead of their frequency since frequency seems to describe more characteristic of color vision independent of the medium?