Like I have Green Rares and Blue Rares... but what i the difference between those and my starting bridge officers that I leveled?

a clean one man bo stinks sorry couldnt resist

to be honest ive had a couple of purples and they were no good to me so go figure

They all start as Ensigns but I think the difference is in the 4 traits that each BO has and the skills they gain as they level up.

Each 'tier' of officers has increasingly better traits and / or access to rarer skills as they level.
That's the big difference... I think ... it's never been explicitly said anywhere but that 's what I've put together.