Speaking from the perspective of someone who prefers the 'historically believable' look:
It all depends on the aircraft! If you're doing a RFC / RAF aircraft, you'd probably want to go with standard PC10 with, maybe, a small name or legend written under the cockpit, or an identifying initial for the upper wing / fuselage side. If you're doing a Central ship however, you can get pretty creative - but pay attention to historical images of the kind of patterns and styles of imagery the Germans used! The Germans were very liberal with their personal markings, but an aircraft that is too complex, or uses colours they might not have had back then, or uses imagery that's too modern, is going to stick out like a sore thumb as a 'fictional' aircraft if you put it in among a line-up of historical aircraft.
Another interesting approach that can generate some inspiration is looking into how and why the Great War pilots marked up their aircraft the way they did - for example, what colours of paints could they readily get their hands on, and what colours were hard to obtain? How was morale at the time? (An alb D.III is probably going to be a lot more gaudy than a Pfalz XII). Did they paint any personal markings, or did they just take what they were given with squadron markings pre-applied? (More likely for Entente services). A while ago I read that one of the reasons the RFC chose to disallow Central-style gaudy designs on their aircraft is because, simply, it made it way easier to tell who was who. "The dull, green ones are our boys, the bright, shiny ones are the Hun"!
(Funny story, I nearly accidentally TK'd a Camel once because he had really brightly coloured plane and I initially mistook him for a Hun because of it!)
For my own personal SPAD XIII I read up on the USAS' orders for markings of aircraft. I then looked at a bunch of real USAS Spads to see where pilots had bent or broken the standard 'rules' for historical markings, and decided which ones I wanted to use for my own XIII