Interesting theory. It would explain less color luminosity differences, but not completely wrong PAL colors (e.g. in PAL Pole Position, the curbs are brown ($44). They could easily be red ($64) and still look OK in SECAM).
I believe that, in case of Pitfall II, the cause of most of the graphic changes is SECAM compatibility.
It would be nice to find some clear evidence or first hand information.