Yellow darkens with Brown as in Yellow Ocher and Mars Yellow.
Brown is dark Yellow.
Ultramarine Blue deepens to a neutral shadow color by adding it's opposite color Yellow,
either a Transparent Indian Yellow Brown/Side or Yellow already deepened to the Burnt Umber hue.

Yellow is the undertone of brown as with the water color pigment of old, Burnt Beechwood Bark,
the organic plants "Turmeric" root and "curcuma" root, from India. Yellow to brown is also stated with the
crystals WAVELLITE and VESUVIANITE and many others.

It takes two transparent oppositions to mix neutral or one transparent and one translucent color,
two translucent colors, or one transparent and one opaque. Two opaque colors just won't do it.

Top-tone is adding White to the color.
Under-tone is adding clear.
Mass-tone is thick out of the tube.
Transparent is clear like colored glass.
Translucent is floating colloidal compounds of colored elements.
Milk is translucent.
Opaque is dense and light blocking, like a rock.
YYYY = As Yellow as you can get,
MMMM = pure Magenta
MMCC = Ultramarine Blue
CCCC = Thalo Cyan Blue

These are the colors to mix to make a neutral dark from Blue and Yellow pigments.
Yellow pigments range from yellow to brown.

WAVELLITE, Al3[PO4]2[OH,F]3.5H2O, H3.5, SG-2.36, orthorhombic crystal system,
acicular, like the needles of Rutile suspended in clear crystal, only these needle crystals form a dense rocklike mass.
ALUMINUM, yellow, brown, blue, centering standard, transparent. Aluminum really does it big this time, each needle starts at the center radiating outward, concentrically ringed bands form around the center, each band ending as a cleavage line, twenty or so bands per complete needle crystal. The band itself can change from clear transparent to opaque white while in the center, the second band range is transparent yellow crystal, than brown, than blue. From brown to blue, the crystal can change into an aggregate and lose it's crystal properties.
THIS MINERAL PHOSPHATE CRYSTAL SHOWS THE DEEPENING OF YELLOW TO BROWN AND
BROWN TO BLUE, LIKE MY REAL COLOR WHEEL FOR PIGMENTS.

NEXT COLOR, Lead Yellow
PREVIOUS, Yellow Green opposite Dioxazine Purple