Thought you might be interested in how I have been playing with backgrounds for paintings on YUPO, as I know a lot of people get scared about such matters. I have a head start as many moons ago I used to use ink acrylics in a similar way, just adding areas of chosen colour for the subject and then spraying with water and leaving at a slight angle to merge and run. Here are three different examples of my YUPO/watercolour experiments:

This first one is simply that, areas of colour with quite a few fine sprays of water left leaning to run ... and I captured the moment when the paint was still moving.

No 2 is taken a stage further so that before it stopped running I used a soft sponge roller very gently.

No 3 is more gentle in colours (ie not so much pigment), sprayed finely in some areas, rolled with sponge in others and used pieces of card to remove colour in square shapes almost back to the YUPO white but just a little staining left to show.
So here is a finished background and floral piece put together. After creating the background, I simply removed the paint with a damp brush in the shapes of the petals and stalks - no drawing at all. Then in places allowed some of the background to merge into the petals by softly spraying selected areas, and in others adding stronger tones of the magnolia pink.

All great fun.
By the way, do you like my replacement photo with my new glasses on My Profile photo???