The majority of metals are known for appearing grey to our eyes, cf. e.g. [Why are most metals gray/silver?](https://physics.stackexchange.com/q/72368/2451) But the main exceptions to this are the "famous" group eleven metals, where their distinctive colours have been adequately explained before due to electron bands and the jump from d to s shells (along with the relativistic effects having a large effect on Au) and whatnot, but where does the "blue-ish" tint of Osmium originate from relative to the rest of the heavy d-block elements? Is it also from relativistic origin or is it sill a mystery? 

Information about Osmium's colour is nearly elusive on the internet and so I'd appreciate an answer, even if it is a glorified "I don't know" presently.