When "Extermination" came out, a lot of people were impressed with the rich azure blue and metallic blues that were seen for the Alpha Legion marines contained therein. The colours of the Alpha Legion have traditionally been many and varied - ranging from purples to blues, to silver, and even green and red in some very old publications! I personally liked the painting of Forge World's contemptor dreadnought for the Alpha Legion and initially wanted to emulate what they had done - but perhaps a bit more moodier or darker.
To achieve this, I tried out a VAST number of recipes, including even using nail polish / nail varnish to try to achieve the tone and level of sparkle that I desired. Let me say for the record that working with nail polish was very tough indeed -- it dries far too fast to be useful and the brushes that they come with are simply no good for detail. So don't do what I did and ruin a paint brush trying it out.
In the end, what I turned to was Vallejo paints. And one in particular: Gunmetal Blue (pictured).
Using my airbrush, I went over the rune fang silver base layer with a thinned down gunmetal blue layer. Note the words: thinned down. In order for the silver layer to (literally) shine through, it needs to be relatively thin. One way to consider this is that its always better to be too thin as then one can apply extra layers on top to thicken it up. But if a thick layer is placed on top, it is impossible to thin it down without starting over again. Hence the advice here is to go thin and do multiple coats if needed.
For the snipers, I applied a thin coat and then doubled up on the lower regions to give a richer colour (darker and a bit less silvered) on the legs compared to the zenith direction. The photo does not quite do it justice, but there is a certain twinkle there that can be seen in the paint.
Whilst I was at it, I also did an Alpha Legion Tartaros terminator (below) to get a better feel for zenith-applied gunmetal blue. The shine on this with the lights is terrific. I really like how they all look, but will now wait until they are all fully dried before proceeding to the next level of detail application.