As promised, here is the additional walkthrough for painting pack marking shoulderpads. I would do them at the same time that I would be painting the yellow shoulderpads.
Step 1 - Paint the shoulderpad interior with GW Evil Suns Scarlet. Nice thin layers will get a nice bright solid base coat.
Step 2 - In the areas to stay red, highlight with GW Wild Rider Red. Follow this up with a top edge highlight of GW Troll Slayer Orange. Once dry, use GW Carrowberg Crimson wash, as a glaze over the red.
Step 4 - Once the red glaze is dry, take GW Abaddon Black and paint the contrasting color for the pack. Highlight the black with P3 Coal Black.
Step 5 - On the pack honor marking, paint the mark with P3 Frostbite. Glaze this with a 50/50 mix of GW Nuln Oil and Drakenhoff Nightshade. Once dry, layer GW White Scar onto the marking to highlight the Frostbite.