This is what I've done so far:
1. I put a "velatura" over the entire painting, of zinc white plus glaze medium. This this translucent layer of white pulls the darks up into medium value range, making the contrast very subtle. Use zinc white, because it's transparent. Do not use titanium white. It's too opaque. In some parts of the painting you can still see this, such as the cat's face.
2. Then, a faint green glaze over the entire piece, wiping off parts of it like the bird, the cat, and the little dragon.
3. I started in on the tree trunk, glazing greens and reds over it, and darkening parts of it. I also put some highlights on the tree trunk with zinc white, impasto.
4. There is a very thin glaze of magenta on the bird. Also the squirrel has been lightened up with some white/gray impasto.
You can go back and forth with thin glazes and impasto. For glazing with acrylics, I use
Matisse "Flow" acrylics (the Jerry's house brand) or Liquitex soft body. I like the Golden glazing medium because it dries slowly and allows you time to smooth in and wipe off an over-all glaze. I also like the Liquitex glaze mediums for glazes that don't have to be smoothed so much or wiped, because these mediums dry fast.
NOTE: Acrylics dry darker, so I add very little color to my glaze medium.
For more about velaturas, read "How to Paint Like the Old Masters" by Joseph Sheppard. He's talking about oils, so you'll have to translate it into acrylics. But it works the same way, to put a velatura over the finished underpainting, and also paint back and forth using impasto for highlights, and thin glazes.