Gets or sets the annotation's colour (C, PDF 1.1): its icon background, pop-up title bar or link border; null when absent, and setting null removes it. Setting it keeps the appearance: call RegenerateAppearance() to draw the new colour.
public PdfDeviceColor? Color { get; set; }
ISO 32000-2 §12.5.2, Table 166. An empty array is transparent; 2 or more than 4 numbers read as absent with a diagnostic.