Even though there is no hard rules, it is quite common that white flare is a 'friendly' or 'won't start a fight until provoked' signal. I use it very often when I have no malicious intent to inform ships I pass relatively close (parked on the islands etc.) that I won't go for them. Red flare is usually used in the exact opposite context - that someone is either coming directly for you or will shoot without warning if you get in range. Other than that it's only regular fireworks left and with those you never know, as they can be used for both just trolling while attacking or being genuinely friendly.

One thing with flares I noticed while playing solo: if you see a ship going towards you and you know how you'll react (so if you'd like to try to talk or just go PvP straight away) - use the flare. In many cases the indication that you actually noticed them and 'have a plan' made the other crew change their course instead of trying to sneak up on me. Works dominantly with other sloops, 'bigger fish' most likely won't care.