From this far away (New Zealand), I'm not 100% sure of anything! :lol
I'm just going on what's available. There's a passage in Famous Spaceships of Fact and Fantasy which goes something like 'The Ties were painted a light blue, almost powder blue, but because gray values were lost in printing the Ties appeared much whiter on movie-house screens'. It may even say a 'light gray-blue' (that's from memory, as I can't find my copy right now).
And that was published in 1979, before TESB, when there was only one movie and it was called Star Wars!
Then there's the bit in SAG, where Lorne Peterson says they weren't allowed any blue on the models, but that "the TIE fighter's Stormy Sea squeaked through thanks to the amount of gray in the mix". I think if you see both gray and blue TIEs on screen in Jedi it has more to do with slight variations in colour timing during compositing.