There actually isn't a gradient in effect. It's just when the image is scaled down like that, much of the detail is lost and it gives the effect of a gradient. Here is the escape pod, blown up so you can see the detail. This is copied straight from the top view of Venture without any changes. So if the image of the Venture were big enough, this is exactly how the pods would appear.
When the drawing is finally done, I'll do some full size sets that will be big enough to clearly show all the detail.