You don't need real paint to achieve this effect. Do it in Photoshop!
1. Open a new project and start with a white background.
2. Choose the Mixer Brush tool (under the regular brush tool)
3. Set the mixer brush option to Wet, Heavy Mix
4. I used the Flat fan high bristle count brush (2nd row down, 6th brush over) but any brush will give similar effects. Experiment!
5. Pick any color and start painting. Keep choosing more colors and overlap to mix.
6. Go crazy. The best part about photoshop is that you can't make any irreversible errors 🙂
7. For a final effect, I added a black background layer and set blend to Difference.