DuracellEnergizer said:

I just noticed a big fail that's bringing this thread down: too many attempts to answer my question using in-universe explanations.

I don't want those, people! You can explain anything in-universe. I want out-of-universe, real world behind-the-scenes answers.

Good question and something I have never given any deeper thought. But I would guess it was an easy way to show the audience who is shooting who in space battles and perhaps an aesthetic reason, looks quite good. But it doesn't answer your question why the regular gun fire is always red on both sides. Maybe the different colors was a very late addition in post which only was applied to space fx shots due to time limit?

I recall the walkers in Empire had blue lasers originally, seen in one of the teaser trailers, probably changed because of the blue sky and the snowy environment just like Luke's saber was made green in Jedi due to the blue sky making it not stand out so well.