0: 0: 0 1: 1
These are not mathematically valid statements. That factor(6) once printed something doesn't make it ok.
Also, since a prime multiplied by a unit is also a prime, these are ok: -30: -2 3 5 -30: 2 -3 5 -30: 2 3 -5 -30: -2 -3 -5 are also valid - although a single -ve prime as the first entry is probably best.
No. At best these are misleading. Unless factor(6) contains explanatory text on prime ideals (it shouldn't), please have it stick with N. It is not a utility for theoretical analysis: it's a utility to print prime factors of "normal" numbers.