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Thanks to MatrixPlot, I can observe some patterns in a matrix. However, when I use MatrixPlot :

MatrixPlot[A, ColorFunction -> "Monochrome"]

The matrix is colored with 3 colors : - black - white - brown

Is it possible to color the matrix only with 2 colors : - black when there is something not null - white when the element is null ?

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dat = RandomChoice[{Null, 0, 5}, {3, 3}];
MatrixPlot[dat, ColorFunction -> "Monochrome"]

Mathematica graphics

You can replace Null with None:

None is interpreted as a missing value and displayed using transparency

MatrixPlot[dat /. Null -> None, ColorFunction -> "Monochrome"]

Or, use ColorRules to display Null as White:

MatrixPlot[dat, ColorFunction -> "Monochrome", ColorRules -> {Null -> White}]

Alternatively, use ColorRules to specify the colors directly,

MatrixPlot[dat, ColorRules -> {Null | 0 -> White, _ -> Black}]

All three approaches above give:

Mathematica graphics

To display Null as White and all other values as Black, use

MatrixPlot[dat, ColorRules -> {Null -> White, _ -> Black}]

Mathematica graphics

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