I started this conversion the other day as a way to see if I could convert my own style of Cataphractii terminator armour. I have to say that painting him has certainly helped with the conversion. I used Forge World's scheme since I really like how the darker green looks. Normally these guys are seen as a much brighter and pale green, but I think the dark green looks pretty sharp.
As far as painting, I started out using the exact same approach I used for my Dark Angels. The green spray basecoat over the black primer. Form there, I added my metallic details and spent a little bit of time working on the face since it is the focal point of the model.
I opted to make his base a light color to contrast against the very dark model. A good discussion on that concept can be found here. I also toned my weathering down considerably on this guy and really tried to make sure I didn't over do it.
The painting does help hide some of the model's imperfections and gives you a much better feel for how he looks in the end. Sometimes it can be hard to imagine how a model is going to look in the end.
I think my favorite aspect is that this guy actually feels Pre-Hersy to me. I like the idea of Space Marine legions and not tiny chapters. There's something about that idea that is more "40k" to me than what Space Marines are today. I'm hoping to get my hands on the new Betrayal book from Forge World to see what else is out there from this period.
As a small side note, I was careful in how I applied my shading washes as well so that I could keep the highlighting effect I had with the spray paint. I think it helps the model much more than just covering the whole thing with a couple washes. It takes a few more minutes since you have to think about where you're applying your shading, but it's worth it in the end.
Make sure to check out these posts as they might help:
Here's the WIP conversion for this model
How to apply an overhead spray basecoat