First, bleach the green out of them. I put them in my stainless steel sink with a mix of about 2/3 water and 1/3 bleach. Some start to lighten right away, and others will take over an hour, but eventually they will all become pretty much off-white. The water turns brown, which is surprising since the trees are green. If you have some really stubborn ones, you might need to drain the bleach water and start over with a new mix. The dirtier the bleach-water gets, the more slowly it will bleach the trees.
Then let them dry for a while. You can rinse them, but it's not necessary. I didn't rinse. They don't have to be completely dry before you color them, but them dry for a while. You definitely don't want bleach dripping off onto your clothes.
They look pretty good like this, so I decided to leave some off-white and I kept one light green one.
When the trees are pretty dry, you can start dyeing them. I used small spray bottles to squirt on a mixture of food coloring and water, but you could also dip them into a glass, vase, or bucket. If you use food coloring, you probably won't want to buy enough to fill a whole bucket, but with Rit dye, a bucket might be appropriate.
The food coloring I used worked great. The problem is that if you handle them later on with wet hands, the color could come off. I have little kids who would love to play with these, but they can only (cleanly) play with the original green ones or with the off-white ones.
When dyeing, wear gloves! Put the food coloring in a little spray bottle with some water and spray until the color is as intense as you'd like. Or follow the directions to mix the Rit dye and dunk the trees in the bucket or pot until they're dark enough. Let them dry completely, and then you can fluff all the branches, wipe off the bases, and set up your display.
I'm sure you could also spray paint these little guys. Just fluff them up first, since they will probably dry really stiff.
Would you make these to decorate your mantle or shelf?