First, I gathered fabrics that I had previously prepared with fusible web. Patterned papers would work just as well, but I like fabric and have a lot on hand. :-)
Then, I added acrylic paint by sponging and splattering. I kept my color palette somewhat tight, using primarily greens, but adding a bit of red, yellow, and dark blue.
I kept adding until it looked festive. I may have gone too far. White brightened things up a bit, and scribbly paints added nice line work. The final touch was a sprinkling of glitter (yes, over the top, but I think appropriate for this instance).
After the fabrics dried overnight, I cut them into strips.
Using a ruffly rotary blade I cut the strips into triangles.
One triangle is fused to the front of a plain greeting card. See, it's a Christmas tree!
The final detail is to sew a straight line down the center of the card as a stylized trunk. I tried metallic thread, but it is notoriously finicky and didn't like sewing through paper, A bright green quilting weight thread worked a treat. I had to guide the cards firmly through the machine as they wanted to slide around. Several layers of masking tape acted as a helpful guide to the right of the presser foot.
Ta da! Finished cards for Christmas. Holiday color with on-hand supplies.