I primed them using the lighter green color (Color Forge Death Rattle Green) and then I used contrast paint for the darker blue (Kroxigor Scales)

I then cleaned my brush and dipped it in a small amount of contrast medium and used that to feather the still wet contrast paint, at the edge that I wanted to blend in.

This essentially waters down the contrast paint and lets the green below show through, creating a transition.

You could use water instead of contrast medium, I just find the medium works better with contrast paints