This was absolutely, positively the hardest challenge for me so far. I spent quite a bit of time trying to color images with my watercolor wonder crayons with poor results. I pulled out some acrylic paints I forgot I had (picked up at Archivers when they were doing a demo and they've sat ever since) but that didn't work. I ended up using my watercolor markers on a flower stamp from Heatfelt Thanks and "huffing" on it to get a watercolor look. I hope I didn't cheat! My card can be found here:
HYCCT1117 Watercolor Heartfelt by muzbysmom - Cards and Paper Crafts at Splitcoaststampers