There are lots of things that define the Horus Heresy, but for me it's the eclectic dreadnoughts that are properly iconic. Epic and Rogue Trader first referenced the names 'Contemptor', 'Deredeo', 'Furibundus' and Forgeworld re-visioned the former taking design hints from the old Epic plastic version. The Contemptor pattern legion dreadnoughts have become synonymous with the era of the great crusade and the Horus heresy, especially within the books and I just can't imagine a legion collection without at least one of the towering veterans in it. For the XIII legion I've included a full cohort.
This is the part where I apologise up front if your question is "what paints did you use to do the ultramarines blue?". Unfortunately I cannot remember as it was a couple of years ago and foolishly did not write it down. I do remember choosing not to record it because I did two things; firstly I used a mix of colours to achieve the base. This is a bad idea if you are doing more than a single piece as it becomes hard to replicate when doing an army and are looking for consistency. Secondly I am certain the base for the colour was one of Games Workshop's old foundation blues which have been long discontinued, so would not be easy or advisable to replicate anyway. It also explains why the two Forgeworld resin Contemptors look significantly different to the plastic Calth version. In many ways this is a lesson in how not to paint an army. Don't misunderstand me, I'm happy with how they came out as a gaming force but know I will struggle in adding further units, so will have to re-interpret. The best I can offer is I believe I used Mordian Blue (I think?) as the base and mixed in Wolf Grey to create highlights. The rest unfortunately is lost to the sands of time.
The weathering however I am more certain on which was Windsor & Newton artists oils, burnt umber for the grime and burnt sienna for the rust streaks. Having learned from my lesson earlier with the blue, I applied the weathering to the dreadnoughts and vehicles at the same time in one sitting. This was a deliberate act to ensure the environmental weathering was applied in a consistent way across the ground forces to help create an unified look and mask the discrepancies in the armour across the force.