Lets start with some data from Optical Transmission in Thin Metal Films where they deposited a range of metals on glass and measured their transmission. Sticking with their data on the noble metals, they see pretty much what one would expect - some changes in shape as the film thickness goes from 0.5 to 4nm of gold, coupled with the clear emergence of the plasmon absorption feature. There are absolutely no atomic absorption lines in the spectrum (completely expected since any intra-gold transitions are at much much higher energies than the visible range of a few eV. (Also note the comparison of Cu, Ag, and Au, clearly seeing the plasmon absorption features in each of them that are close together, but close in the visible range and thus we are highly sensitive to small differences).
Recall that plasmons are a feature of bulk (not atomic) systems with free carriers. In the noble metals the conduction bands are made up of the outer electrons that are donated to the cause of crystal binding. The plasmon frequency depends on the density of free carriers and their effective mass - that is pretty much all. All of the noble metals have highly similar band structure and Fermi surfaces (both bulk, not atomic, features). A peek at a table of electron binding energies (such as at LBL) shows you that the outermost electron of Au has a binding energy of 57.2eV. The next one is 74.2eV. For comparison, Ag has 58.3 and 63.7 eV, and Cu 75.1 and 73.3 eV, respectively. So, really, the outer electrons of Au are not particularly different from the outer electrons of the other noble metals as one would expect from their band structures noted in your first link.
But, remember that the nice picture we keep in our heads of hydrogen-like atomic orbitals is, well, wrong (except for one-electron atoms, sort of). Bethe and Salpeter wrote an entire book (Quantum Mechanics of One- and Two-Electron Atoms, and I count myself extremely lucky to have taken graduate quantum mechanics from Ed Salpeter) on the difficulties of adding that second electron. With more and more electrons being added, the difficulties mount rapidly because of screening and other factors. Sometimes I sit in awe and contemplate just how well the hydrogen-like orbitals survive all of that.
So, where do 'relativistic effects' come in to play? Looking back at the electron binding energy tables, one sees that the Au 1s electron has a binding energy of 80.725keV, about 15% of the rest energy of an electron. What this means is that, should you want to do a full-up multi-electron calculation of all 79 electron states, you are going to have to deal with the relativistic effects of those core orbitals which will have an influence on the outer states because they have a screening effect different from the simple hydrogen levels. All that means, is that the last few outer electrons of Au see a slightly different potential than they would have if there were no relativity. In the end, given the binding energies for Au compared with Cu and Ag there really is not much difference. The color differences of them are all done to the plasmon absorption in the visible, a bulk feature, with slight different plasmon energies for Cu, Ag, and Au. So, look to electron density and effective mass for your answer.