Blue contains all the content P99 will ever have.
Green is the progression server that is currently about mid velious. As such it's slightly more popular because people enjoy the fresh server and less saturated economy.
Highly recommend starting on green.
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Please don't give up, Project 1999 can be very difficult but also very rewarding the further you get. 1-20 is probably the hardest part. After that your class of choice should open up and enable you to really explore. Don't give up & socialize along the way, every single character you see is a real life person!