Gets the group colour space, the blending colour space of the group's content, or null when the group blends
in its parent's (always so for a non-isolated group). The page group's space without a CS is the backend's.
public PdfColorSpace? ColorSpace { get; }
ISO 32000-2 §11.6.6, Table 145 (CS), §11.3.4 and §11.7.2: colours painted into the group are converted to it and
composite there; device spaces are subject to DefaultColorSpaces. A space that cannot blend (Lab, Indexed,
Pattern, Separation, DeviceN, an ICC profile that is not gray, RGB or CMYK) is not handed over: the group blends in its
parent's space.