The green was intentionally faded using a MIG filter. The filter I am currently using is "brown for dark yellow". Its basically just a tan filter. A filter is somewhat the reverse of a wash, and reduces the contrast of a model. The rust effect is MIG's enamel based brown wash. It if applied to the corners and low points of the model, and it flows all around the cracks and details.
To get the green color, I painted the area knarloc green to give a nice foundation. I then mixed orkhide Green with snot green in a 50/50 mix and added in some reaper flow improver to get the final tone. Flow improver is a good product which I highly recommend if you want a smooth finish. The model isn't done yet, since I will be adding more weathering effects and more painted detail, so stay tuned for more updates.
Also, don't forget about the giveaway I am running which can be found here