All opposites on the color wheel will make brown when combined. Note that every opposite on the color wheel is made up of a combination of the three primary colors which when mixed together makes brown. Interesting how when using pigments the totality of colors equals black and the absence of colors equals white. When using light the totality of colors equals white and the absence of color equals black. Because we can only experience color in the presence of light (a combination of reflection and absorption), we can never actually achieve black using pigments unless the pigments are black and even that is subjective--black always has a red shade or blue shade or green shade etc. subtle though it may be, And white always has a blue shade or yellow shade and so on. And the bottom line of that for our uses is that dark combination of opposite colors on the wheel for sandback produces cleaner dark areas in the contrast than using black pigments.
That is due to the combination of the colors you chose--those opposites on the color wheel and the intensity (saturation--dye to solvent ratio) of dye applied--particularly the darkest one, and just as important, the amount of sandback you did. Very nicely done!
SR