Glad you guys all like it. Looking back on it there are lots of things I would do differently but I was on a slight time limit.
I particularly like the shape of the rust spots. I think considering the size of the rust spots that they seem too smooth. When I think of large areas of rust like that I tend to think of more texture within the rust patches and also more color variation. This could be a little bit more of a red/orange/brown stipple in the interior of the the largest rust areas, or in spots where the rust would have been abraded.
I think the marine's head is too close in color to the body armour paint. So there isn't quite enough separation between the flesh and the armour.
The verdigris also isn't really working for me. It seems like it's layered in mostly for contrast but it looks pretty smooth and flat. On the studs of the powerfist and on the backpack is ends up looking more like bad OSL than oxidation. I'm a firm believer than verdigris should usually be painted with a brush because it is created by the action of water on a surface which an airbrush doesn't usually capture. Verdigris is also not that smooth unless the entire object has been submerged, there's a lot more striation.
On the base plate I did a lot more work on the rust. Before hairspraying I stippled rhinos hide and ryza rust to create more variation in colour but neglected to on the marine to save time.
A few people have said the same thing regarding the flesh. The lighting isn't fantastic on this pic but it is actually a lot more green than it appears. However I probably should have gone a bit more green in retrospect just to make it stand out.
The verdigris was actually done as an afterthought as I thought there wasn't enough contrast, and it was done with a brush rather than the airbrush. It was entirely rushed and I should have taken more care into putting it into the recesses and varying the intensity for a more realistic effect.
Thank you very much for the suggestions. I am actually working on an Age of Sigmar Nurgle warband and and want to use the same colour scheme and effects, so I will bear this all in mind when doing them.