For palette colors, returns the 0-based index of the color in the spreadsheet's color palette. For custom colors, this property will return a negative number. Automatic colors will return an index greater than or equal to 56 (the maximum index in the palette).

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 public int PaletteIndex{ get; }
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<p>Public ReadOnly Property PaletteIndex() As Integer</p>
int colorIndex = clr.PaletteIndex;
Dim colorIndex As Integer = clr.PaletteIndex