Combination #1
Acid Primary Gradient + Ecto Secondary Gradient
Combination #2
Magnetite or Demolition Primary Gradient + Aurora Secondary Gradient
Combination #3
Ivory Primary Gradient + Cobra Secondary Gradient
Combination #4
Titan Primary Gradient + Beetle Secondary Gradient
Combination #5
Shaman or Cobra Primary Gradient + Neptune Secondary Gradient
These are the most accessible and compatible that I have in mind right now, but my favorite is Void Primary Gradient + Demon Secondary Gradient.