Returns whether or not the color is a custom color. For .xls files this means that the color is not part of Excel's default color palette. For .xlsx files, this means that the color has not been retrieved from the legacy palette (through Palette.SetColorAt or Palette.GetColorAt), and is not a system color.

 public boolean IsCustomColor{ get; }

Public ReadOnly Property IsCustomColor() As Boolean

bool customColor = clr.IsCustomColor;
Dim customColor As Boolean = clr.IsCustomColor