It can be challenging working on the smaller size of ATCs, which are 2.5"x 3.5" (or 63.5mm x 90mm). Some techniques transfer well to the miniature size, but others do not. This technique works really well as you are only working with a hand-drawn image and then filling in the negative space with a single watercolour. By using only one colour for the negative space in the background, your eye is immediately drawn to the image, and you can keep this drawing quite simple but still have it look very effective.
"Potential side effects"
"Keep life simple"
"... and breathe"
"Nature"

These are what made me try out this particular technique on
ATCs, but I can no longer find the video I saw. I first
created these on this larger size, which I believe would work
really well on a card.
I knew I should have saved the video so I could link it here.
Apologies to the creator for not linking it, I do love your technique!