Digital painting

There are lots of videos on creeating a colour palette from an image and loading it as swatches.

https://www.youtube.com/watch?v=fX9leGkxtq8

https://www.youtube.com/watch?v=rUFh0JoTj4U

You should also check out Color.Adobe.Com which used to be the Colour Theme extension which disappeared a while back.

 https://color.adobe.com/

This has a colour wheel, as below, but you can also drag and drop an image into it to create a palette from that image.  

There are lots of videos on how to use it:

https://www.youtube.com/results?search_query=color.adobe.com

You can put two docs side by side on screen and use one as a mixing pallete with the mixer brush

Dave

The Mixer Brush tool already does that. You can paint color on your document, get another color, paint over the previous color and they will mix like real paints. How they mix depends on how you set up the Mixer Brush in the options bar at the top (Wet brush? Dry brush? High or low paint load?)

To get the effect you want, you should also select a brush preset or customize the brush options so that you are painting with a round brush, fan brush, sharp tip, whatever.

Photoshop's strong point is Photo editing and composition. A dedicated Paint program would be better.

https://www.painterartist.com/en/product/painter/

If that is out of the questiion,

Photoshop has a mixer brush tool. Here's a video on it: https://www.youtube.com/watch?v=jWx_nSdki1o

Anastasiy offers a color mixing panel, it's been around for a while.

https://anastasiy.com/mixcolors

You can go to the Ideas forum and put in your suggestion, but no guarantees if and when it will happen,

https://community.adobe.com/t5/photoshop-ecosystem/ct-p/ct-photoshop?page=1&sort=latest_replies&filter=all&lang=all&tabid=ideas

Please show the full workspace including layers panel in order to find what's going on.