First submission, nice. :) Here's my pre-Heresy Imperial Fists veteran sergeant, one of the old figures I really like. I'm more pleased with how this looks now than as a WIP, which I guess is a good thing.
He's a member of Sigismund's company, which is why he's got the Templar cross on his right shoulder pad. Both that and the Fist are freehanded. I went for a minimal palette of black, white and yellow for contrast.
As for how the yellow is painted... the short answer is in a time-consuming fashion! Which makes the prospect of painting a battle company as I'd wanted to not so appealing. Since I have too many figures to paint already, we'll see how that turns out.
I started from black and then basecoated Iyanden Darksun. This was then overpainted with Sunburst Yellow, before giving a wash with Gryphonne Sepia. When dry, I highlighted with a mix of Sunburst Yellow and Dheneb Stone. This tones down the brightness of the yellow somewhat. Finally, I painted Yellow Ink over the top, and washed the recessed with a 1:1 mixture of Chesnut and Black ink that I keep in a pot.
The resulting base tone is similar to Iyanden Darksun, so I'm thinking of skipping the Sunburst Yellow in the future and just using Iyanden Darksun and Dheneb Stone for the basecoat and highlighting. The yellow ink adds a lot of yellow colour into the mix, so I don't think the end result will be much different, since Sunburst Yellow doesn't cover so well anyway.
Hope you enjoy!