Colorless, light grey, wispy white
At idle there should be really no visible smoke. A little light gray or very wispy whitish gray during engine warm-up, but nothing resembling bluish/black belching smoke.
Also if you run your motor hard and push your boat into heavy chop or wind, your working your diesel and you will get some black smoke or soot on the transom, no matter how clean your injectors are. Also, any fouling on the prop will cause the motor to work harder and result in more smoke. Any pinhole leaks in your fulel line (air) will add smoke. As long as it isn't noticeable at idle or belching smoke under high load, bluish, very white, or very black smoke, I would n't worry about a little smoke.
If you liked my advice/response, give me some rep points. If you didn't......well what do you expect, it was free.