Actually, according to the rules it would seem that the logical orientation would be yellow front, green bottom.
4d) Cube puzzles must be scrambled with the white (or the lightest colour by default) face on top and green (or the darkest adjacent face by default) on the front.
However, it's really quite unclear, because in the typical orientation (pyraminx resting on table), there is no top face for pyraminx. Calling the front "top" and the bottom "front" seems like a logical extrapolation for pyraminx, though. Any other opinions?