Mixing colours on a screen means averaging RGB, which is why blue and yellow give you grey there and green everywhere else. Here every colour goes into Mixbox’s latent pigment space first. Blue dragged through yellow turns green. White pulls a colour back to a tint instead of just brightening it.
The palette is real paint. Each masstone keeps its Colour Index name, so cadmium yellow is PY35 and phthalo blue is PB15, the same codes on the side of the tube.
Every pigment also carries a tinting strength, and that’s the part that catches you. Phthalo blue sits at 3, titanium white at 0.5. A little phthalo swallows a lot of white, so half and half on the canvas is nothing like half and half in the result.
Pick two and drag one through the other.