Gets the colour table of an Indexed image as stored (m bytes per colour, m the base space's component count), read when the list is built so that a backend never reads the file for it; empty for any other image.
public ReadOnlySpan<byte> ColorTable { get; }
ISO 32000-2 §8.6.6.3: each byte maps onto its base component's range. A backend needs it where it works in the base space, such as removing a Matte from the table's colours (§11.6.5.2).