I'm going to say that ANH tie fighters were grey and in ESB they changed it to blue. It's a very good point that when you pump up the saturation on the GOUT, colour restores to things that looked greyscale previously--the Death Star is another big change that happens once you restore it's colour (that is, mainly blue-tinted). But other things happen too, and you see some of the flaws in the opticals. The reason ties in ANH look blue is the same reason X-Wings look green and pink in the same amount of shots. It's just what happens when you do optical composites--it never looks exactly right. In some shots, the "black" space has also printed blue, especially in cockpit shots. I think it's kind of neat to have these subtle mismatches in the film, you appreciate how hard it was to do these opticals.

In ESB though, the blue is so pronounced and consistent it's clear they were doing it on purpose. In fact, maybe the whole ANH thing was part of their reasoning--since the tie's were printing as blue half the time maybe it would be better to just do that on purpose. Or maybe seeing the look of the blue-printed tie's inspired them to colour them differently in the sequel. If I remember right they went back to the grey colour for ROTJ, the opticals seem better quality so maybe that was why.