I would do these by lifting color or softening color by applying something that is drier. Wet follows dry.
If the darker paint is still wet/damp:
- fold a paper towel/tissue and touch it along the edge, letting it soak up color, OR
- run a dry brush along the edge to pick up the color
You can also pick up less color, by using a damp brush that is drier than the paint.
If the paint has dried:
Run a damp brush along the edge to wet it. Gently squeeze water from the brush with a towel or cloth or switch to a dry brush – the brush must be drier than the newly wetted area. Run it along the same edge, so that the now wet paint will flow to the brush. It’s a little trickier when the paint has dried, and you might have to go through the process more than once.
There are several factors than can affect the outcome. Some paints are staining and you might be able to lighten a little, but not much. Some papers don’t allow you to lift well. They absorb the color too much and won’t let go or they may just turn to mush and get icky. And, brushes vary in how much water they hold – so some will always be too wet or too dry.
This can be a tricky thing to do, and may take practice.
There may be other methods, but this is the only one I know. I hope it helps.