I am sorry to be a spoil sport but RYB for mixing will not do “realistic life like mixing” your seeking.
If you read this thread you will understand it more Hardmode - Manually Mix Colors This because when you mix paints there is alot more going on than just RYB or CMYK.
Either way how to mix RYB in Pigmento:
- press the triangle for the settings dialog
- activate “MIXER” to show the mixer sliders
- change to Mixer = RYB to swap into the RYB mixer
- Select a color 1 > Ctrl+LMB on one of the boxes
- Select color 2 > Ctrl+LMB on the box of the other side.
- Click on the slider in the middle to adjust the amount from one to the other
After your done just ALT+LMB to clean because it will not forget the colors until you clean.