Higher quality officers have slightly better traits (more Superior traits and less Basic traits), but it makes a pretty minor difference to be honest. Almost all traits are ground only, and the ones I can think of that work in space don't have weaker and stronger forms like some traits do.
The main difference of boff quality is powers. Rarer boffs are more likely to have level 3 skills that aren't normally obtainable.
thanks for the answer, all of them are usefull. However, now I have another question: I have several BO candidates, one is violet, others are blue and others are green. The color of my BO candidates has any impact like in duty officers? If yes, do you think is better to discard the BO I have and skill some of my acutal colored BO candidates?
Well kind of.
First: Usually the green purple and blue Bridge officers come with abilitys wich cant be purchased directly from the skill trainer or be trained by the captain himself.
Torpedo Spread III, Scattern Valley III, Tykens Rift III, Targert Subsystem X III, Beam Overload III Transfer shield strenth III are famous examples for that..
Those abiltys CAN be trained to other BOs (wich "consumes" the BO that traines, means he disapears).
They also come with better traits, wich usually just effects their preformance on Ground and is not that important (especially if you play in groups and rarly take BOs with you anyway)
In some very rare cases you get a saurian with efficiency improoves the enegry levels in space and is very desired (although, IMO, a litte overestimatet).
Violet (called Purple here in the forums) = VERY RARE. You probably don't want to throw them away.
Blue = RARE. Generally worth keeping
Green = UNCOMMON.
Very Rare bridge officers will have *two* Superior Traits (for example, an Andorian might have Superior Aggression and Superior Cold Resistance).
Rare bridge officers will have *one* Superior Trait. Uncommon ones have . . . well, I don't remember what they have, because I usually get rid of them in favor of Rare or Very Rare ones.
There are times when it might be unwise to get rid of an existing bridge officer, though. If you have a well-trained Common or Uncommon (white or green) officer, you will LOSE the points you've put into them if you send them away. It might be wiser to retain the old officer, not put any further points into him or her, and then *train up* a Rare or Very Rare officer that you plan to put into their position in the future. Then, when the new officer is better trained, you can get rid of the old one and put the new one in.
As has been stated, better officers can also have special skills, possibly making them more valuable and useful. You should consider how an officer can fit into your plans before investing points in them.
WARNING -- take all pieces of equipment off of the obsolete bridge officer BEFORE you send them away. They will take the equipment with them and you won't get it back. This can suck if they had good gear on them. I found this out the hard way.
Open up the crew roster and go to the 'passives' tab. You will see specific trait for each Boff, the vast majority of which only effect ground combat. Better quality Boffs will have more/better traits. The difference, however, is not huge. While leveling it is more important to spend points making sure that you have a full roster of ground/space powers. Only replace Boffs with better quality Boffs if you have extra skill points to spend. Once you hit Captain and above you'll start having extra Boff skill points and thus more freedom to replace Boffs.