Currently, the cnc painter uses the same brush cleaning technique that human canvas painters use. Swirl the brush around in a cup of water and wipe it off on a paper towel. This procedure is performed at the start of any painting operation to ensure that the brush bristles are wet enough to accept paint, periodically during the painting process to prevent paint from drying on the brush, between paint colors, and at the end of the painting to clean the paint off the brush in preparation for storage. I would classify this technique as semi-autonomous. You don't have to physically clean the brushes yourself, but you do have to replace the water and paper towel periodically during operation. This technique has worked great so far, but has a few downsides -> the paper towel takes up a large percentage of the workspace (leaving less space for the canvas) and different brush sizes and types require unique water swirling and towel wiping patterns to effectively clean the paint of the brush.
I tried to come up with a universal mechanical solution. It involved rotating a brush while spraying it with powerful jets of water. I bought a small 12Vdc pump and designed a 3D printed nozzle that would direct two streams of water (90 degrees apart) at the same location. In order to avoid changing water as often, the water used to clean the brushes would be recirculated and a large volume of water would be used to dilute the paint that was washed off the brushes. It turned out working quite well for a variety of brush sizes and types. The water jets were strong enough to deflect the bristles and clean the center of the brush. The only additional design consideration would be to make an enclosure for it. Some of the water was splashing into my face.
Still looking around for more options I purchased a cheap ultrasonic cleaner bath. Even after one minute there was usually still a little bit of paint left on most brushes. It also gave me a headache if I left it on too long with the lid open.
Thinking the inverse of the ultrasonic cleaner may be more effective (vibrating the brush instead of the water), I bought a sonicare toothbrush next. It did about as well if not a little worse than the ultrasonic cleaner bath.