The team at Addicted to CAS is playing with just one layer for challenge #218. To me, a one layer card means there is no other cardstock on the card other than the card itself. You can add a few embellishments (pearls if you need to) but there shouldn't be any layers or dies (unless you do an inlaid technique). It's a difficult thing to do... and it's hard to get depth and movement.
I saw a card on Pinterest a while back that I decided to try as a one layer card. The card I saw had this awesome wave of colour, but it was paper strips that were then covered by layers of white cardstock to create the wave. I had to put my thinking cap on to try to figure out how to make it just one layer.
To create it, I cut a "stencil" with a piece of printer paper using a wave die I have in my stash. I cut 1/4" off the length of it and adhered the two pieces to my card front with green tape from Scrapbook.com. I then used sponge daubers (the finger tip ones) and added my rainbow of Tim Holtz inks to the wave. I let the coloured overlap because it would have been way too difficult to not have them overlapped LOL. I added some Nuvo Aqua Shimmer to the wave and stamped my sentiment so it overlapped the wave just a bit.