All good advice and plain white is fine but the truth is that these trailers will get stolen in a heartbeat if not in an extremely secure location and a plain white one is preferable to a thief. It's harder to ID going down the road as he leaves and does not require a complete paint job immediately before selling, etc.
Obviously, you would not want a mural of a cool 911 on both sides but trailers like that are worth stealing all day long
for the trailer. Whatever is inside is just a bonus. I'd definitely do a colored stripe or two, maybe two tone just for visibility on the highway plus making it distinctive for lookouts when it gets stolen. But that's just me.
