Gets the colour space the shading's colours are interpolated in, and the space of every colour this model stores or EvaluateFunction(ReadOnlySpan<Single>, Span<Single>) returns: ColorSpace, or the base space of an Indexed colour space.
public PdfColorSpace InterpolationColorSpace { get; }
ISO 32000-2 §8.7.4.4: Indexed colour values "shall be immediately converted to the base colour space".