Here's an explanation of the Green Leaves, Red Leaves and Custom User Titles.

The leaves represent "ranks" and in AQ, it depends solely on your post count.
1 x : 10 to 29 posts.
2 x : 30 to 49 posts.
3 x : 50 to 199 posts.
4 x : 200 to 499 posts.
5 x : 500 to 999 posts.
5 x : 1000 and more posts.

User titles
There are 2 types of user titles. Assigned and custom. Assigned user titles are automatically given to members when they reach certain post counts:
Junior 0
Member 10
Regular 30
Veteran 100

However, members who reach 30 post counts, have the privilege of using custom user titles. To customise your user title, go to your User CP and select Edit Profile. If you are allowed to customise your user title, you will see a section where you can edit your user title.

Do note, promotions to higher tier membership on the forum is performed automatically by the forum software on an hourly basis. So, you might not be promoted at the very instant that you acheive the necessary post count. Do wait at least a day (just in case) before you panic and pester the Administrator (yours truely) about your inability to change your user title or access certain privileges. This also applies to privileges such as larger PM mailboxes and access to restricted forums like the Marketplace forums.