The list[1] shows who you trust and who you do not trust. There is a default list of people you trust. This list appears by default for all people in this forum and can be modified by adding or removing people "~username".
The people in that list can change your trust as their feedback is reliable.
You need two positive trusts and wait for more than 10 months to get a dark green color.

[1] https://bitcointalk.org/index.php?action=trust;full