Interesting. Without very odd paints, I can’t see how you’d get “yellow”, but if your red is desaturated (scattering and not totally absorbing some blue through yellow), and if your white is a bit gray, but your white absorbs and scatters red more than it absorbs and scatters the rest of the spectrum, the mixing path through Lab* color space would (I think) go from red toward light orange, toward warm light gray, finally coming into the light-gray color from the orange direction. I’m a little rusty, but I think that’s right.