Greenies are reputation points for posting good info. When you are a new member, you have fifty points, and one green square. You can give "green" to others, but it really doesn't add any point value to thier rating, and your "green" will show up on thier CP as blue. Once you've been given enough green by others to increase your value to 100 points, you will get a second green square next to your avatar, and the greenies you give to others will increase thier points value.
Make sense??
Also, the higher up you go, the more greenies it takes to get to the next level. And the higher up you are, the more points your greens-to-others are worth.
Hope this explains it!!