I’ll take a stab at that right off the cuff... 69/70 when we qualified for Europe I think was the green and black one and was our best away season ever (in the top division anyway), and 87/88 I think was our best away season since then and that was the yellow Hummel one. Add this season and I’m gonna go for Hummel yellow as our most successful away strip. But that’s completely going from memory, no research done.
Id love a Roma style maroon away kit... Red has been shown to offer psychological benefits in the form of increased aggression and looking scarier and more dominant to the opposition.