Yellow Green is an opaque color, Sap Green permanent is basically the same color transparent but lower on the chroma scale because it has some black added.
Mix it with it's opposite color transparent Dioxazine Purple for a neutral dark.

It takes two transparent oppositions to mix neutral or one transparent and one translucent color,
or 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,
YYYC = Yellow Green
YYMM = Yellow and Magenta, as Red as you can get.
MMMM = pure Magenta
MMMC = Purple
CCCC = Thalo Cyan Blue
 

These are the Yellow Green to Purple opposition pigments that make a dark neutral color

      PY83 stable di-arylide + PG7 chlorinated copper phthalocyanine = Sap Green Y/S
      PY83 stable di-arylide HR + PG7 chlorinated copper phthalocyanine + PO43 perinone orange = Sap Green O/S
      PY129 methin copper complex = Green Gold
      PY?.. azomethine = Genuine Green Gold
PY129 methin copper complex = Golden Green, Indian Yellow Green with PY153

36.  CYYY, Old Holland Yellow-Green Organic Opaque, 35_10_36
35.  Winsor and Newton Sap Green permanent, Transparent, 35_1_35

3.  MMMC, Liquitex Carbazole Dioxazine Purple, Transparent, 15_1_23
24.  Grumbacher Dioxazine Purple, Transparent, 15_1_24

PV23:1r carbazole dioxazine = Purple
 

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