These are marketing names for different product series. Green are advertised as environmental friendly, power saving models while Black are the high-performance models. Blue is between.
The different colors are just different models. There is green, blue, black, and red I believe. Green is for less power consumption, blue is the basic drive, black is a faster drive, and red is for performance. The prices go up with each color.
Generally the front headphone jack will override whatever is in the back. So for me, I have my speakers plugged into the back and they play my music. When I plug in my headphones, the speakers don't play and my headphones do. Therefore, you can't have both playing at the same time. There may be a way to pick which one plays in your settings but I don't know without looking.
You can separate audio channels and leave your headphones plugged in but use your speakers. I don't remember exactly how I did it but in the sound settings where all your audio devices are you can split them. Unfortunately you have to go back into your devices every time you want to switch and disable one device and enable the other.