Why doesn't oil paint mix like colors on a screen?
Short answer: Screens add light, while paint subtracts it: each pigment absorbs and scatters light, so mixtures behave in ways RGB math cannot predict. Models such as Kubelka-Munk describe how real pigments absorb and scatter, which is why they predict oil mixes far better than digital color pickers.
Light versus pigment
A monitor makes color by adding red, green and blue light. Mix blue and yellow light and you get something near white. Mix blue and yellow paint and you get green, because each pigment is absorbing parts of the light the other reflects.
Absorption and scattering
Every pigment both absorbs some wavelengths and scatters light back. Titanium white scatters strongly; ultramarine absorbs strongly and scatters little. The Kubelka-Munk model treats these two properties separately, which is why it captures things like white cooling a warm color or a small amount of a strong pigment dominating a mix.
What this means in practice
A color picker can tell you what a spot on a photo looks like on screen. It cannot, on its own, tell you which tubes and in what proportion will get you there in oil. Tools built on pigment behavior close that gap. Easel Edge, built at Clint Atelier, uses Kubelka-Munk mixing for exactly this reason. The painting itself is still made by hand.