Gets or sets the group colour space (CS): the space colours are converted into when painted into the group, the blending
space, and the space of the group as a whole; null, the default, inherits the parent group's or page's.
public PdfColorSpace? ColorSpace { get; set; }
ISO 32000-2 §11.6.6, Table 145 (CS), and §11.3.4. Ordinarily present only for isolated groups and page groups; it shall
be present in the group of a luminosity soft mask (§11.6.5.1), which has no parent to inherit from. Setting records the space's
minimum version; null removes the entry. A mutation.