Frequencies of light are objective and measurable.
Oh, boy, you picked the *wrong* analogy
Neither "sky blue" nor "midnight blue" is fully characterized by a single wavelength of light. Both are names for *human experiences*, not single wavelengths of light. Midnight blue, for example, is actually more about intensity than wavelength, it is about the *lack* of light. If it is mostly dark, and kinda bluish, you'll call it midnight blue.
Moreover, the human eye is a strange thing. It isn't like it has receptors for each and every wavelength. We typically have three types of photo receptors, which each respond differently to lights of different wavelengths - one is most sensitive in the blue range, one in green, and one in yellow. Color perception starts by comparing the relative strengths of responses of the three types. It is like having three variables (S, M, and L), and any responses that give the same set of values of S, M, and L, will look like the same color, to us. And there are different ways to achieve that. So, there are many different sets of photons that we would call "sky blue", rather than one wavelength we can unambiguously call "sky blue".
On the other hand, we have two things that are clearly similar (God) but which have some differences in definition (trinitarian vs unitarian).
Um, well, that one is sticky - we have to things that appear similar to an outsider, but which are *reported* as having different qualities (like trinitarian vs unitarian). Reports cannot be naively taken to be definitions.
Are these the same thing, distinguished only because of difference in perception, or are these different things because the definitions are different?
I don't have an answer to that. All I'm saying is that it's valid to ask the question.
Is it valid to ask? I mean, the answers are generally non-falsifiable, so what's the point of asking, other than to note that we can't be sure of the answer?