Gets the default colour spaces of the current resources, which replace the device spaces of what is painted now: pass them to GetColorConverter(PdfColorSpace, PdfDefaultColorSpaces?) with the colour being painted.
public PdfDefaultColorSpaces DefaultColorSpaces { get; }
ISO 32000-2 §8.6.5.6: the look-up uses the resources current when the object is painted, so inside a form XObject the form's own defaults apply. The initial DeviceGray colour is remapped too.