How the trust system works:
The trust system isn't heavily moderated, anyone can leave anyone else feedback, you do not need to trade or anything.
Due to this, some feedback can be fake. To prevent this, we use a trust list. Each user has a list of people who they think leave other users "real", accurate feedback, called a trust list. When you view someone elses trust page or score, you will only see feedback from people who are in your trust list.
By default, users start with a list called DefaultTrust. You can add or remove people here:
https://bitcointalk.org/index.php?action=trustAdding or removing people only changes the way scores appear to you, not anyone else. Other people who look at your trust page will see a different score depending on who they have in their trust list. Someone might have a user who left you positive in their trust list and then see a green score, however someone else could have a user who left you negative in their list and see a red score.
So to answer your question, the reason you do not have a green score is you are (probably) looking at your profile with DefaultTrust and do not have have positive feedback from anyone on DefaultTrust. Someone else who looks at your profile with a different trust list might see red,green, orange etc.
