Well, I finished painting Mortarion. I'm happy with the results, although I have to admit that I found the whole process rather difficult. It was mostly just getting the colours to look good that I struggled with. My own style tends to be quite vivid, I like bright reds, blues and purples, but Mortarion is a sort of creamy off-white. It wasn't difficult as such, but more that I found it difficult to think it looked "good" as it isn't a colour I would personally choose. To compensate for this and to make him look a bit more interesting I used the full range of Death Guard Colours (adding in green and red compared to the Forgeworld one). I'm particularly happy with the freehand numerals on his armour.
On the subject of his armour, it was also difficult to match the colour of his armour with a good colour for the base - too light and he'd just blend straight in. Equally though it had to be muted so that your eye would be drawn to the model and not the rock he was standing on. In the end I went with a stone effect for the top and just tried to keep the rest fairly dark.
For those who missed it, I also painted Roboute Guilliman (Primarch of the Ultramarines). You can read all about it here:
That article contains a fairly comprehensive painting guide for him too if you're interested in the kind of techniques I use.