Wasn't it the case that the models used in the first film were grey but were repainted blue for ESB?

If that's the case one could argue that the blue fighters come from the blue Executor and the grey ones come from the grey Star Destroyers and the grey Death Star.

The colour could be a base-ship identifying mark.

It might make sense after a battle to be able to visually identify which damaged or wrecked fighters were from the command ship and which ones were deployed from the other fleet ships.

Doing that would maintain design features from the original film but give a logical rationale for the design difference.

In which case you could keep the grey fighters in ANH, have most of the fighters blue in ESB and have a mix of grey and blue in ROTJ.