Just a quick demonstration of the New Simple Water brush on Painter 12.
Some things I forgot to mention in the video:
The brush will blend like a dream as long as you are painting alla prima, but if you close the file and reopen on another day...not so much. There's probably a way around that but I just don't know.
You can add lighter colours on top of darker colours as well, so long as it's within the alla prima stage of the painting.
If you want to blend it, you can either add a new layer and make sure you are 'Picking up Underlying Colour' and blend away (recommend the Grainy Water) or you can blend it to your hearts content after it has left the alla prima stage.
Here's the sketch in case you want to try it out