I think that might be quite difficult, as from what I have seen, a lot of hybrids come in all sorts of shades of green. In my own limited collection of orchids, where I have got 5 phals in one window, every single one of them is a different colour some with and some without blotches (can't think of the technical term just now) but definitely with some being much darker than others.
I think a lot depends on parentage and what species they descend from. With the more complex hybrids, and most phals that I am going to have are so far removed from their species origins that they could be called uber-complex, I don't know if there would be even a chance of working out what the leaf colour should be.
If you are working with primary hybrids or species I imagine that this would be a lot easier.
Generally where most people refer to leaf colour and light, I think there is an assumption that you are comparing what the leaf colour is now to when it was bought, on the basis that if bought from a reputable vendor it should have been in ideal conditions and therefore the colour would have been in it's ideal range.
Does that help a bit? - I am sure others will chime in if I am talking a load of rubbish.