I would think there would be a lot of trees made. Lot of fun images come to mind to applique using the theme of Green. Green as green with envy, Green as green thumb, Green as in greenhorn, Green as in green eating (cows eating grass), Green as in moon made of green cheese.

You could do blocks (whatever you decide on) in different types of fabrics, if you have access to older clothing would be even better. The green theme would be reduce reuse recycle.
A theme, rather than a color, although if all the fabrics were green, well then you've gone above and beyond :o)

A tree. A shamrock. A cloverleaf. Leaves. A vine with leaves and flowers. Embroider members names on the leaves and/or petals.

Hmmm .... Kermit the frog sings "It's not easy being Green" ... how about some frog and Lily pad appliqué?

A couple of years ago a hand dye fabric company put out a challenge to make a small quilt with an acid green hand dye. The theme of the challenge was the Broadway show "Wicked". I was so fortunate that I got to see the display of 90% of the challenge quilts at the Syracuse AQS show. the remaining 10% were travelling with the off Broadway production of the show, picked by cast and crew members. Here is a google link to images of the challenge. I bet you will get loads of ideas from this:
https://www.google.com/search?q=wick...ifBbgQ_AUICCgD

I did a Shakespeare In The Park quilt all in green.

http://www.quiltingboard.com/picture...s-t224391.html

How much work do you want to put into this project? How much will you use the finished project? Make something that pleases you and don't use precious time that might not be of value/use to you in the future. Could just use the color in a favorite design. Sometimes challenge pieces only fill the bill for the project, not our own desires!

There are a number of patterns using trees. Could maybe incorporate the tree idea into something for Christmas? My first thought though was something to celebrate St Patrick's Day, coming right around.

Great size for a table runner or wall hanging. Lots of great ideas presented too. Good luck. I did a challenge with my guild and it was actually a great experience to think outside the box (and my comfort zone) for a project that I didn't have to invest a lot of time or money into.

What about a birthday cake in the middle with green icing and candles and balloon on both sides or a big applique "10" on one side. Whatever you do be sure and share pictures with us. If you just used the cake and balloons then you could use it as a runner or banner for family birthdays. Ideas are popping in my head. You could do "Happy Birthday" in the center with balloons and or cake/cupcakes on the sides.

Of course, my first thought was something using the "green eggs and ham" from Dr. Seuss. It might make a cute table runner for someone with small kids, along with a copy of the book...