I had one such bag that I've been meaning to bleach and use for colored bottle brush trees for quite some time but the project keeps getting put off.
I used 4 ounces of bleach and 6 ounces of water. I soaked the trees in the mixture for 15 minutes. When they looked an off white color, l pulled them out, rinsed in hot water, and brushed off the painted snow. You'll have to pick a few stubborn pieces of paint off.
Let them dry.
When they are fully dry, mix a combination of about four ounces of vinegar, 3 or 4 drops of food coloring, and six ounces of water. Dip the trees into the mixture and swish them around. Leave them in the mixture for 2 or 3 minutes, or however long it takes to get the color saturation you desire. They will color quickly. I used some colors that were left over from a Halloween neon color kit.
Now you can decorate them and add them your other Christmas decor, or save them to use in a Christmas project. Such as the upcoming Christmas in July Putz Workshop.