I’ve found this reference for a measurement of the nucleus by the HST
http://adsabs.harvard.edu/abs/1998A&A…335L..25L
“ The color of the nucleus is moderately red with a gradient of 10% per 1000 Angstroms at optical wavelengths”
which is consistent with my measurement of the reflectance spectrum of the inner region. This is not proof that we are seeing the nucleus of course but the similarity is interesting
Robin